Iron phosphate and preparation process thereof

Abstract

The invention belongs to the technical field of lithium ion battery materials, and discloses iron phosphate and a preparation process thereof, and the preparation process comprises the following steps: oxidizing a ferrous solution to obtain a ferric iron solution; the pH value of the ferric iron solution is 1.6-2.0; adding a phosphorus source into the ferric iron solution to obtain a pretreated iron source solution; the molar weight ratio of P in the added phosphorus source to Fe in the ferric iron solution is 0.10-0.60; conveying the pretreated iron source solution, phosphorus source solution and ammonia water into a reaction kettle in a parallel flow manner, and reacting to synthesize iron phosphate. By adding the phosphorus source solution into the ferric iron solution, the problem of scaling of a pipeline for conveying the iron source solution is well solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and in particular relates to iron phosphate, a precursor material of lithium iron phosphate, and its preparation. Background Art

[0002] Iron phosphate is the primary precursor material for lithium iron phosphate (LFP), and has a direct impact on the performance, quality, and cost of LFP cathode materials. Producing iron phosphate with stable quality, good uniformity, and low price has always been a hot topic in the lithium battery industry.

[0003] The common ammonium ferric phosphate synthesis process uses ferrous sulfate as the iron source, oxidizing ferrous iron to ferric iron. The ferric iron then reacts with a phosphorus source and ammonia water to synthesize ferric phosphate. However, after long-term production operation, it was found that the oxidized ferric iron easily hydrolyzes to produce ferric hydroxide colloid, which adheres to the delivery pipeline, valves and tank walls, resulting in inaccurate measurement by the flow meter on the pipeline, and the pipeline delivery volume cannot meet the production capacity requirements. It may even lead to iron source loss, resulting in an imbalance in the raw material ratio and ultimately unstable finished product indicators. Finding a method that can inhibit the hydrolysis of the ferric iron source, solve the scaling problem of the delivery pipeline and ancillary equipment, and reduce the loss of iron source is an important technical difficulty in stabilizing ferric phosphate production. Summary of the Invention

[0004] In response to the problems existing in the prior art, in a first aspect, the present invention provides a process for preparing iron phosphate; in a second aspect, the present invention provides iron phosphate.

[0005] Specifically, the present invention provides the following technical solutions.

[0006] A process for preparing ferric phosphate, comprising: (1) Oxidizing the ferrous solution to obtain a ferric solution; the pH value of the ferric solution is 1.6-2.0; (2) adding a phosphorus source to the ferric iron solution to obtain a pretreated iron source solution; the molar ratio of P in the added phosphorus source to Fe in the ferric iron solution is 0.10-0.60; (3) The pretreated iron source solution, phosphorus source solution, and ammonia water are transported to the reactor in parallel for reaction to synthesize iron phosphate.

[0007] In a further preferred embodiment, the oxidant used to oxidize the ferrous solution is liquid oxygen.

[0008] In a further preferred embodiment, the ferrous solution is a ferrous sulfate solution.

[0009] In a further preferred embodiment, the phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0010] In a further preferred embodiment, the phosphorus source is added to the ferric iron source solution through a static mixer.

[0011] In a further preferred embodiment, before the iron source solution is transported to the reactor, the iron source solution is transported to an oxidation storage tank and continuously stirred in the oxidation storage tank.

[0012] In a further preferred embodiment, the stirring speed in the oxidation storage tank is 20-40 rpm and the temperature does not exceed 50°C.

[0013] Ferric phosphate is prepared by the above preparation process.

[0014] In a further preferred embodiment, the specific surface area of the iron phosphate is 5.7-7.4 m 2 / g.

[0015] Compared with the prior art, the above one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: The invention adds a phosphorus source solution into a trivalent iron solution, thereby effectively solving the scaling problem of a pipeline for conveying the iron source solution.

[0016] The static mixer device is used to mix the trivalent iron source solution and the added phosphorus source solution, thereby ensuring mixing uniformity and preventing local precipitation at the interface between the iron source and the phosphorus source.

[0017] A small amount of phosphorus source is added to the ferric iron source solution to prevent iron loss, ensure that the P / Fe ratio in the reaction system is more stable, and obtain good product indicators of ferric phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The scaling conditions of the iron source solutions after oxidation in Examples 1 to 5 and Comparative Examples 1 to 4 are shown; Figure 2 This is a SEM image of the ferric phosphate product obtained in Example 1; Figure 3 This is a SEM image of the finished ferric phosphate product obtained in Comparative Example 1; Figure 4-Figure 7 The following are SEM images of the finished ferric phosphate products obtained in Examples 2 to 5; Figures 8-10 The SEM images of the finished ferric phosphate products prepared in Comparative Examples 2 to 4 are shown; Figure 11 These are UV-visible absorption spectra of the pretreated iron solution obtained in Example 1, the iron solution in Comparative Example 1, the ferric sulfate solution, and the synthesized amorphous ferric phosphate. DETAILED DESCRIPTION

[0019] The currently used ammonium ferric phosphate process uses ferrous sulfate as an iron source to oxidize a ferrous solution into a ferric solution. This ferric solution is then mixed with a phosphorus source solution and ammonia in three parallel streams to synthesize ferric phosphate. However, after long-term production operation, the pipeline transporting the ferric solution was found to have a serious scaling problem. Sampling and analysis confirmed that the scaling material was ferric hydroxide.

[0020] After oxidation, trivalent iron can easily hydrolyze to produce ferric hydroxide colloid, which adheres to the conveying pipeline, valves and storage tank walls, resulting in inaccurate measurement by the flow meter on the pipeline, the pipeline delivery volume cannot meet the production capacity requirements, and may even lead to iron source loss, resulting in an imbalance in the raw material ratio and ultimately unstable finished product indicators.

[0021] Scaling of pipelines transporting ferric iron source solutions is a common problem, mainly due to the hydrolysis of ferric iron to produce ferric hydroxide colloid and the precipitation of impurity ions in the raw materials, which form a hard scale layer on the inner wall of the pipeline.

[0022] The applicant's research found that the higher the temperature of the transported ferric iron source solution and the higher the system pH value, the more severe the scaling and the faster the scaling rate. However, since the ferrous raw material needs to be heated and dissolved, and heat is released during the oxidation process, the overall temperature of the ferric iron solution is 48-58°C, which is difficult to stabilize at a lower temperature. In addition, in the existing production process, the pH value of the ferrous solution is generally 0.8-1.3. The pH of the ferrous solution is relatively low, but the oxidation process consumes hydrogen ions, which increases the pH value of the oxidized ferric iron source solution (the pH value generally increases to 1.6-2.0), which also aggravates the problem of pipeline scaling. In addition, the existing process system makes it difficult to reduce the pH of the oxidized ferric iron source solution. The formation of scale on the inner wall of the pipe by the ferric iron source solution reduces the cross-sectional area of the flow channel, reducing the pipeline transmission efficiency. Secondly, it will increase the flow resistance, increase the power consumption of the pump or fan, and shorten the equipment life. In addition, the shutdown and cleaning caused by scaling reduces the continuous operation period of the equipment, resulting in a decrease in product output. For scaled pipes and equipment, the current treatment method is to use a combination of chemical cleaning and mechanical cleaning, add other reagents to dissolve the scaling materials, and then use high-pressure water jet cleaning. In addition, because some iron is lost in the form of scale, it is impossible to accurately control the P / Fe ratio of the product during the batching process, or it requires great effort to control it.

[0023] To solve the above problems, some embodiments of the present application provide a process for preparing iron phosphate, comprising: (1) Oxidizing ferrous solution to obtain ferric solution; the pH value of the ferric solution is 1.6-2.0; (2) adding a phosphorus source solution to the ferric iron solution to obtain a pretreated iron source solution; the molar ratio of P in the added phosphorus source solution to iron in the ferric iron solution is 0.10-0.60; (3) The pretreated iron source solution, phosphorus source solution, and ammonia water are transported to the reactor in parallel for reaction to synthesize iron phosphate.

[0024] The present study found that adding a small amount of phosphorus source to the ferric iron solution can make the ferric iron preferentially combine with phosphate to form ferric phosphate, thereby inhibiting the iron in the ferric iron solution from forming scaling substances such as ferric hydroxide. The UV-visible absorption spectrum characterization ( Figure 11 ) It can be seen that the pretreated iron source solution obtained in Example 1 provided by the present invention and the amorphous ferric phosphate exhibit very similar absorption peaks. It is speculated that adding a small amount of phosphorus source to the ferric iron solution can form the same UV-visible light absorption peak as the amorphous ferric phosphate; and after the addition of the phosphorus source, no other precipitate is generated in the ferric iron solution. The phosphorus source added to the ferric iron source is the same as the phosphorus source for the subsequent synthesis of ferric phosphate, which can further ensure that the P / Fe ratio in the reaction system is stable and unchanged, and the early addition of some phosphorus source will not affect the product indicators of the ferric phosphate.

[0025] Further research in the present application found that as a phosphorus source is added to the ferric iron source solution, the pH value of the ferric iron source solution decreases, which can further reduce the risk of ferric iron precipitation and scaling.

[0026] Further research in this application found that when the molar ratio of P in the added phosphorus source solution to iron in the ferric iron solution is within the range of 0.10-0.60, the more phosphorus source is added, the more amorphous iron phosphate is generated, and the better the effect of inhibiting hydrolysis. If the P / Fe molar ratio in the pretreated iron source solution is higher than 0.6, it is more likely to cause the iron phosphate to precipitate directly under the same pH conditions, and the change in the feeding method is likely to cause the product specific surface area to be low; if the P / Fe molar ratio in the pretreated iron source solution is lower than 0.10, the amount of amorphous iron phosphate generated is too small to effectively inhibit the hydrolysis of ferric ions.

[0027] In some preferred embodiments, the oxidant used to oxidize the ferrous solution is liquid oxygen. Compared to using hydrogen peroxide as an oxidant, using liquid oxygen as an oxidant is less expensive, and the pH change of the solution before and after oxidation is consistent with that using hydrogen peroxide, which does not affect scaling.

[0028] In a further preferred embodiment, the phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0029] In a further preferred embodiment, the phosphorus source is added to the ferric iron source solution via a static mixer. By adding a small amount of the phosphorus source to the ferric iron source solution via the static mixer, the two streams of material undergo diversion, cross-mixing, and counter-swirl without the need for external force, allowing the added material to diffuse rapidly and evenly throughout the system, achieving instantaneous mixing and effectively preventing the formation of local precipitation due to localized over-concentration caused by instantaneous collision of the two streams of material.

[0030] In a further preferred embodiment, before the pretreated iron source solution is transported to the reactor, the pretreated iron source solution is transported to an oxidation storage tank and continuously stirred in the oxidation storage tank.

[0031] In a further preferred embodiment, the stirring speed in the oxidation storage tank is 20-40 rpm and the temperature does not exceed 50°C.

[0032] Ferric phosphate is prepared by the above preparation process.

[0033] In a further preferred embodiment, the specific surface area of the iron phosphate is 5.7-7.4 m 2 / g.

[0034] The present invention will be described in more comprehensive and detailed manner below in conjunction with the accompanying drawings and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0035] Example 1 (1) Preparation of ammonium dihydrogen phosphate solution: Weigh 59.26 g of ammonium dihydrogen phosphate into a beaker, add 166.41 g of water, stir and dissolve to obtain ammonium dihydrogen phosphate solution with a mass concentration of 6.9% phosphorus; (2) Preparation of ferrous sulfate solution: Weigh 700 g of ferrous sulfate solid, add 800 g of deionized water and stir for 10 min to pre-dissolve, add 5% concentrated sulfuric acid to prepare ferrous sulfate stock solution, the mass concentration of which is 7%, and then add compressed oxygen to the ferrous sulfate stock solution into a sealed pressure-resistant reactor. The pressure in the reactor is controlled at 0.5-1.0 MPa. The solution is fully reacted in the sealed pressure-resistant reactor until the pressure in the reactor drops to a constant value. The oxidized trivalent iron solution is filtered to obtain a mass concentration of 6.31% iron and a pH value of 1.853. (3) Pretreatment of ferric iron solution: 238.04 g of oxidized ferric iron solution was taken, and 18.33 g of ammonium dihydrogen phosphate solution with a phosphorus concentration of 6.9% was added while stirring, according to a molar ratio of P to Fe of 0.152. The mixture was stirred for 10 min to obtain a pretreated ferric iron solution. (4) Synthesis of ferric phosphate: The pretreated ferric iron solution, 117.70 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia were mixed in parallel to prepare ferric phosphate.

[0036] Comparative Example 1 (1) Preparation of ammonium dihydrogen phosphate solution: Weigh 59.26 g of ammonium dihydrogen phosphate into a beaker, add 166.41 g of water, stir and dissolve to obtain an ammonium dihydrogen phosphate solution with a mass concentration of 6.9% phosphorus; (2) Preparation of ferrous sulfate solution: Weigh 700 g of ferrous sulfate solid, add 800 g of deionized water and stir for 10 min to pre-dissolve, add 5% concentrated sulfuric acid to prepare ferrous sulfate stock solution, the mass concentration of which is 7%, then add compressed oxygen to the ferrous sulfate stock solution and put it into a sealed pressure-resistant reactor. The pressure in the reactor is controlled at 0.5-1.0 MPa. The reaction is fully carried out in the sealed pressure-resistant reactor until the pressure in the reactor drops to a constant value. Filter to obtain the oxidized iron source solution; the mass concentration of iron is measured to be 6.31%, and the pH value is 1.853; (3) Synthesis of ferric phosphate: 238.04 g of the oxidized ferric iron solution obtained above, 136.03 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia were mixed and flowed in parallel to prepare ferric phosphate.

[0037] Example 2 The difference between Example 2 and Example 1 is that: In step (2), in the pretreatment of the ferric iron solution, 27.61 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.230, and the mixture was stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.276; In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 108.42 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0038] Example 3 The only difference between Example 3 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 36.90 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.307, and the mixture was stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.259. In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 99.14 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0039] Example 4 The only difference between Example 4 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 45.94 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.382, and the mixture was stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.110. In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 90.09 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0040] Example 5 The only difference between Example 5 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 55.23 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.459, and the mixture was stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.076. In step (4) of synthesizing ferric phosphate, the pretreated trivalent iron solution, 80.81 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0041] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 9.28 g of ammonium dihydrogen phosphate solution with a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.077, and stirred for 10 minutes to obtain a pretreated ferric iron solution with a pH of 1.642. In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 126.75 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0042] Comparative Example 3 The difference between Comparative Example 2 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 107.12 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% was added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 0.890, and the mixture was stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.119. In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 28.92 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0043] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that in step (2), in the pretreatment of the ferric iron solution, 122.75 g of ammonium dihydrogen phosphate solution having a phosphorus concentration of 6.9% is added to the oxidized ferric iron solution while stirring, according to a molar ratio of P to Fe of 1.02, and the mixture is stirred for 10 minutes to obtain a pretreated ferric iron solution having a pH of 1.003. In step (4) of synthesizing ferric phosphate, the pretreated ferric iron solution, 13.28 g of ammonium dihydrogen phosphate solution, and 26 g of aqueous ammonia are flowed in parallel to prepare ferric phosphate.

[0044] Figure 1 The scaling of the iron source solution after oxidation in Examples 1 to 5 and Comparative Examples 1 to 4 is shown. Obviously, adding a small amount of phosphorus source to the ferric iron solution can inhibit the iron in the ferric iron solution from being produced as scaling substances such as ferric hydroxide, and as the amount of phosphorus source added to the ferric iron solution increases, the effect of preventing scaling becomes more obvious. In Comparative Examples 3 and 4, the molar ratio of P in the phosphorus source added to the ferric iron solution is greater than 0.60. Although scaling does not occur, the performance of the obtained ferric phosphate is affected. The inventors further combined the above results to find out the scaling of the iron source solution after oxidation. Figure 2-Figure 10 , and analyze the physical and chemical characteristics of iron phosphate.

[0045] Figure 2 This is a SEM image of the ferric phosphate product obtained in Example 1. Figure 4-Figure 7 The following are SEM images of the ferric phosphate products obtained in Examples 2 to 5: Figure 3 This is the SEM image of the ferric phosphate product obtained in Comparative Example 1. Figures 8-10 These are SEM images of the finished ferric phosphate products obtained in Comparative Examples 2 to 4.

[0046] Figure 11 These are UV-visible absorption spectra of the pretreated iron solution obtained in Example 1, the iron solution in Comparative Example 1, the ferric sulfate solution, and the synthesized amorphous ferric phosphate.

[0047] The Fe / P content of the ferric phosphate obtained in Examples 1 to 5 and Comparative Examples 1 to 4 was determined by titration, which is a common method in the industry. The particle size of the ferric phosphate was determined using a Bettersize 2600 laser particle size distribution analyzer. The physicochemical characteristics of the ferric phosphate are shown in Table 1.

[0048] Table 1 The above data show that there is no significant difference in the Fe / P and sulfur content impurities of the pretreated iron solution obtained by adding some phosphorus source to the trivalent iron solution in advance, and the BET shows an upward trend. However, after the phosphorus source is added in excess in advance, the BET drops and the particle size shows signs of increasing. This may be because the addition of a small amount of phosphorus source to the trivalent iron solution can inhibit the hydrolysis of trivalent iron and prevent the production of ferric hydroxide colloid. The presence of ferric hydroxide colloid will cause the nucleation sites of ferric phosphate to adhere to its surface at the beginning of the synthesis reaction, making the particle size larger. Therefore, the addition of phosphorus source can reduce the particle size to a certain extent and improve the specific surface area. As the amount of phosphorus source added increases, the pH of the pretreated iron solution obtained will decrease to a lower level. At this time, the initial pH of the system will decrease when participating in the synthesis reaction. More energy is required to reach the pH of ferric phosphate precipitation, and the time of the entire reaction is consistent, so the reaction will be more intense per unit time, resulting in uneven growth of ferric phosphate particles, ultimately leading to large particle size and reduced specific surface area. This can also be confirmed from the SEM image.

[0049] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A process for preparing ferric phosphate, characterized in that: include: (1) Oxidizing the ferrous solution to obtain a ferric solution; the pH value of the ferric solution is 1.6-2.0; (2) adding a phosphorus source to the ferric iron solution to obtain a pretreated iron source solution; the molar ratio of P in the added phosphorus source to Fe in the ferric iron solution is 0.10-0.60; (3) The pretreated iron source solution, phosphorus source solution, and ammonia water are transported to the reactor in parallel for reaction to synthesize iron phosphate.

2. The process for preparing ferric phosphate according to claim 1, wherein: The oxidant used to oxidize the ferrous solution is liquid oxygen.

3. The process for preparing ferric phosphate according to claim 1, wherein: The phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

4. The process for preparing ferric phosphate according to claim 1 or 3, wherein: The phosphorus source is added to the ferric iron source solution through a static mixer.

5. The process for preparing ferric phosphate according to claim 1, wherein: Before the pretreated iron source solution is transported to the reactor, the pretreated iron source solution is transported to an oxidation storage tank and continuously stirred in the oxidation storage tank.

6. The process for preparing ferric phosphate according to claim 1, wherein: The stirring speed in the oxidation storage tank is 20-40 rpm and the temperature does not exceed 50°C.

7. A ferric phosphate, characterized in that: Prepared by the preparation process according to any one of claims 1 to 6.

8. The ferric phosphate according to claim 7, wherein The specific surface area of the iron phosphate is 5.7-7.4m 2 / g.

Images