Method for preparing battery-grade iron phosphate by using titanium dioxide by-product ferrous chloride

Abstract

The invention discloses a method for preparing battery-grade iron phosphate by using titanium dioxide by-product ferrous chloride, and relates to the technical field of production of lithium iron phosphate precursors. Comprising the following steps: preparing a ferrous chloride crude product into a solution, adjusting the pH value of the solution, adopting an operation sequence of oxidizing ferrous ions while synthesizing the ferric phosphate during synthesis of the ferric phosphate, so that the conversion rate of the ferrous ions is high, the growth process of the ferric phosphate crystal is convenient to control, and then adjusting the pH value and the temperature again in the aging stage of the ferric phosphate crystal to obtain the ferric phosphate crystal. The influence of impurities in the iron phosphate is further reduced, so that the iron phosphate crude product is gradually converted into qualified iron phosphate. According to the method, in the process of preparing the battery-grade iron phosphate from the titanium dioxide by-product ferrous chloride, the influence of impurities in the by-product on synthesis and preparation can be reduced by adjusting the pH value of the corresponding solution for multiple times and optimizing the adding sequence of the reagents, so that the finished iron phosphate contains few impurities and is high in purity, the utilization rate of the titanium dioxide by-product ferrous chloride is increased, and the method is suitable for industrial production. The cost is saved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery material preparation, and in particular, relates to the technical field of production of lithium iron phosphate precursors. Background Art

[0002] Among the constituent materials of lithium-ion batteries, the positive electrode material is the source of lithium ions that can be intercalated and deintercalated in the entire battery, and directly determines the key performance of lithium-ion batteries, such as energy density and safety. At present, commercial lithium-ion battery positive electrode materials are mainly divided into lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate and nickel cobalt manganese (aluminum) oxide lithium ternary materials. Among them, lithium iron phosphate has significant advantages in safety performance and cycle life, and has become a hot topic of concern. In the mainstream two-step synthesis of lithium iron phosphate, the control of the preparation quality of the precursor iron phosphate has a direct impact on the subsequent electrochemical properties of lithium iron phosphate. One of the key indicators is the molar ratio of iron to phosphorus in the product. In traditional processes, iron salts and phosphoric acid are usually reacted to prepare iron phosphate, or iron salts and phosphates are reacted to prepare iron phosphate. These processes often require a pure iron source as a raw material and cannot achieve good resource recycling.

[0003] Titanium dioxide, as a white pigment with the best performance, is widely used in coatings, plastics, papermaking, printing and rubber industries. The domestic production of titanium dioxide is mainly through the sulfuric acid process or titanium chloride. Among them, the production process of titanium dioxide chloride will produce a large amount of by-products whose main component is ferrous chloride. 2+ 、Al 3+ , Mn 2+ Mg 2+ Impurities such as chlorination and condensation make it impossible to be effectively utilized, which not only causes a waste of resources, but also greatly restricts the development of the chloride titanium dioxide industry.

[0004] The Chinese patent with application number CN202211623008.4 discloses a method for synthesizing an iron phosphate precursor from ferrous chloride, a byproduct of titanium dioxide, comprising: S1, reacting a solution of ferrous chloride, a byproduct of titanium dioxide, with an alkaline solution to obtain a solution A; S2, adding an acid regulator and an oxidant to solution A to obtain a solution B; S3, adding a phosphorus source to solution B to obtain a solution C; S4, adding the solution C and an alkaline regulator to a reaction vessel pre-filled with a small amount of water as a base liquid, and after the reaction is completed, the iron phosphate precursor product is obtained by post-treatment. The patent proposes that the iron phosphate precursor can be synthesized from the byproduct of titanium dioxide ferrous chloride, and the quality indicators of the obtained iron phosphate can meet the requirements of the "HG / T4701-2014 Iron Phosphate for Batteries", and the utilization rate of iron in the titanium dioxide byproduct can be high without introducing new impurities, and ions such as chromium, aluminum, titanium, and vanadium can be better removed. However, the application mainly involves hydrolyzing and removing impurities from the ferrous chloride solution by adding a large amount of alkali regulator. This operation will result in a large amount of ferrous ion precipitation loss, resulting in poor recycling of the titanium dioxide byproduct ferrous chloride into battery-grade ferric phosphate and a low conversion rate.

[0005] In summary, although the above patents disclose that ferrous chloride, a byproduct of titanium dioxide, can be used to prepare battery-grade iron phosphate, the preparation process is to simply hydrolyze and remove impurities in the ferrous chloride solution and then add a phosphorus source for oxidation, which will cause the loss of ferrous ions. At the same time, due to the presence of more impurity metals in the titanium dioxide byproduct, the conversion of ferrous ions to iron phosphate will inevitably be affected during the preparation process, thereby affecting the yield and morphology of iron phosphate. Therefore, how to reduce the impact of impurities in titanium dioxide byproducts so that they can be better converted into qualified battery materials and realize the resource application of byproducts in the titanium dioxide production process has become a technical problem that needs to be solved by those skilled in the art. Summary of the invention

[0006] In view of the problems in the prior art that ferrous chloride, a by-product of titanium dioxide, often needs to be treated with alkali to remove impurities, resulting in loss of ferrous ions, causing low conversion rate and poor morphology of the finished ferric phosphate, the present invention provides a method that can reduce the influence of impurity elements in the titanium dioxide by-product and increase the utilization rate of ferrous chloride therein, so as to prepare battery-grade ferric phosphate with better morphology.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] A method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, comprises the following steps:

[0009] S1: crystal impurity removal: dissolving the crude ferrous chloride product and adjusting the pH value to 4-4.3 to obtain a ferrous chloride solution, wherein the content of ferrous chloride in the crude ferrous chloride product is 254-320 g / L.

[0010] S2: preparing a phosphorus source: preparing a phosphorus source solution, adjusting the pH value to 8-8.5, and mixing with hydrogen peroxide to obtain a mixed solution.

[0011] S3: preparing crude ferric phosphate: passing the above mixed solution into the above ferrous chloride solution and stirring to obtain crude ferric phosphate.

[0012] S4: Crystallization and aging: Add hydrochloric acid to the above crude ferric phosphate and adjust the pH to 0.8-1, and age at 92-95° C. to obtain a ferric phosphate solution.

[0013] S5: Preparation of iron phosphate: The iron phosphate solution is filtered, washed, dried and calcined to obtain battery-grade iron phosphate.

[0014] Generally speaking, the byproduct of the production of titanium dioxide by the chlorination process is treated initially to obtain a crude ferrous chloride product, at which the content of ferrous chloride in the crude product is about 254-320 g / L. It is dissolved in water again to prepare a ferrous chloride solution. At this time, if alkali is added to hydrolyze the impurities, due to the high content of iron ions in the solution, a large amount of iron ions will combine with hydroxide to form a precipitate, and the impurity removal effect is often not very significant, and a large amount of ferrous ions will also be lost. Secondly, the crude ferrous chloride product is prepared into a solution to prepare ferric phosphate. At this time, the solution still contains some impurity ions. Even if the content of impurities in the final ferric phosphate product is controlled to be qualified, the presence of impurities in the process of synthesizing ferrous chloride into ferric phosphate will still affect the morphology of the ferric phosphate particles, making the particle size of the particles uncontrollable.

[0015] In the present application, the crude ferrous chloride is prepared into a solution, and the ferrous ion content in the solution can be made appropriate by adjusting the pH value of the solution, so that more iron phosphate is generated in the subsequent synthesis reaction. After the phosphorus source solution is prepared, it is blended with an oxidant, and a method for synthesizing iron phosphate while oxidizing ferrous ions is adopted when synthesizing iron phosphate, which can make the conversion rate of ferrous ions higher while facilitating the control of the growth process of iron phosphate crystals, and facilitate the regulation of particle size. Compared with first adding an oxidant to obtain iron chloride and then reacting with a phosphorus source for synthesis, it is possible to avoid mixing other impurity ions in the process of synthesizing crude iron phosphate. Afterwards, the pH value and temperature are adjusted again in the aging stage of the iron phosphate crystal, and the influence of impurities in the iron phosphate is further reduced, so that the crude iron phosphate is gradually converted into qualified iron phosphate, and the morphology of the iron phosphate is better while the conversion rate can reach 99.99%.

[0016] Specifically, in S1, the pH value after the crude ferrous chloride is dissolved is adjusted to 4 to 4.3, which can control the reaction process while maintaining a good content of ferrous ions, preferably 110 to 140 g / L. When the pH value is greater than 4.3, the ferrous ions will continue to react and form precipitation, resulting in a decrease in the content of ferrous ions in the solution; if the pH value is less than 4, the initial pH value of the subsequent reaction is low, the iron phosphate precipitates slowly and dissolves, affecting the yield.

[0017] After that, the soluble phosphorus source is prepared into a phosphorus source solution, and the pH value is adjusted to 8 to 8.5. The acid-base environment at the end point of the reaction can be regulated to ensure that the iron phosphate is basically precipitated. In addition, the present application adds hydrogen peroxide to the phosphorus source during the stage of configuring the phosphorus source and mixes it evenly. Compared with directly adding an oxidant for oxidation, adding an oxidant to the phosphorus source solution can reduce the concentration of the oxidant, which is convenient for controlling the growth particle size of the primary particles of iron phosphate. In addition, hydrogen peroxide is selected as the oxidant, and the oxidation intensity is moderate and will not bring in excess impurities. After dilution with the phosphorus source solution, no additional process water is required, and the concentration of ferrous chloride will not decrease, which also relatively improves the yield of iron phosphate.

[0018] Preferably, the addition of an alkali regulator such as ammonia water, diammonium hydrogen phosphate or ammonium dihydrogen phosphate can moderately regulate the pH value of the phosphorus source solution so that the pH fluctuation is not too large. Compared with the direct use of the phosphorus source, the phosphorus source and the alkali regulator are prepared and used, which can reduce costs, and by regulating the pH value, the entire preparation process has high reproducibility, is relatively stable and easy to obtain.

[0019] Preferably, the phosphorus source is selected from common ones: disodium hydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid, which can be well mixed with the alkali regulator to form a phosphorus source solution with a suitable pH value.

[0020] After solution configuration is complete, the mixed solution of phosphorus source solution and hydrogen peroxide is passed into the reactor equipped with the above-mentioned ferrous chloride solution and stirred, and crude iron phosphate is synthesized by reaction. Different from prior art, ferrous phosphate is synthesized after oxidizing ferrous iron first, the mixed solution of the present application has both the phosphorus source required for reaction and the hydrogen peroxide required for oxidizing ferrous iron, and the oxidation of ferrous iron and the crystallization of ferrous phosphate are carried out synchronously during the reaction, and the pH value therein is controlled and adjusted respectively based on the previous preparation of solution, in the synthesis stage of S3, ferrous ions are oxidized to ferric iron immediately after contacting the oxidant in the above-mentioned mixed solution, and then continue to react with the phosphate therein to generate ferric phosphate, and under the corresponding acid-base environment, the ferrous ions in the ferrous chloride solution can be mostly converted into ferric phosphate crystal particles for precipitation, and then reduce the removal of foreign ions. In the process, stirring continuously, while the reaction is carried out smoothly, the growth particle size of the primary particles of ferrous phosphate can be well controlled, and the finished ferrous phosphate with uniform particle size distribution can be obtained.

[0021] Preferably, the reaction in S3 can be carried out at room temperature of 5 to 25°C. Since the solution environment is suitable before, the reaction can proceed smoothly, and the synthesis stage of the present application can be carried out without high temperature, which reduces energy consumption and further reduces costs. Controlling the stirring speed at 300 to 400 rpm can make the particle size of the crude iron phosphate particles more fine and uniform, and obtain a more uniform finished iron phosphate precursor after subsequent calcination.

[0022] After the crude iron phosphate is synthesized, hydrochloric acid is added to adjust the pH of the solution to 0.8-1. At this time, the iron phosphate particles are in the aging stage. In this stage, the crystal particles are in a dynamic equilibrium process. In the process of repeated precipitation, dissolution, and re-precipitation, the crystals gradually grow and purify. Controlling the pH can further dissolve the impurities in the finished iron phosphate and reduce the content of impurities in the finished product. If the aging temperature is regulated at 92-95°C, the crude iron phosphate can be converted into qualified iron phosphate after repeated dissolution and precipitation, and the conversion rate can reach 99.99%. If the temperature is too low at this time, the conversion is too slow. If the temperature is too high, the iron phosphate particles will clump.

[0023] After aging, the above iron phosphate solution is filtered, washed, dried and calcined through S5 to obtain battery-grade iron phosphate. The battery-grade iron phosphate obtained in this application has very few impurities, of which the manganese content is only about 13ppm, the physical phase is pure, and the iron phosphate particles have uniform particle size distribution, the D50 value is 6.5-8.5μm, the iron-phosphorus ratio is stable at 0.96-0.975, and the morphology is uniform. It is a high-quality precursor for preparing lithium iron phosphate battery materials.

[0024] Preferably, a vacuum filter or filter press is used in the above filtration, which can achieve better filtering effect for solid-liquid mixtures with high solid content and fine particles. The above washing includes: using ultrapure water to wash the conductivity of the filtrate to 200μs / cm, which can well control the impurity content in the finished ferric phosphate product and slightly affect the iron-phosphorus ratio, and drying the solid at 70-120°C after washing, and calcining at 300-700°C, so that the ferric phosphate particles are sintered evenly without agglomeration.

[0025] Compared with the prior art, the technical solution of the present application has at least the following beneficial effects:

[0026] 1. In the process of preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, the present invention can reduce the influence of impurities in the byproduct on the preparation by repeatedly adjusting the pH value of the corresponding solution, so that the impurities in the finished ferric phosphate are less and the purity is better, and the yield of preparing ferric phosphate from ferrous chloride, a byproduct of titanium dioxide, is improved.

[0027] 2. When the reaction of synthesizing the crude iron phosphate of the present invention occurs, the ferrous ions react with phosphate immediately after oxidation to precipitate iron phosphate, and the oxidation and crystallization processes are carried out simultaneously, which can control the growth particle size of the iron phosphate particles, thereby obtaining a finished iron phosphate product with uniform particle size and element distribution.

[0028] 3. The present invention uses ferrous chloride in the titanium dioxide byproduct to prepare the iron phosphate precursor, which realizes the recycling of resources, is conducive to cost saving and improves resource utilization. In addition, the reaction of the present invention can be carried out at room temperature, which is easy to control and the process operation is simple, the wastewater can be recycled, and it is easy to industrialize and apply; the prepared iron phosphate has uniform particle size distribution, pure phase, and high cleanliness, and is an excellent precursor for preparing high iron phosphate lithium battery materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments and comparative examples. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 This is a SEM image of the battery-grade iron phosphate precursor prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0032] S1: crystal impurity removal: dissolving the crude ferrous chloride product and adjusting the pH value to 4-4.3 to obtain a ferrous chloride solution, wherein the content of ferrous chloride in the crude ferrous chloride product is 254 g / L-320 g / L.

[0033] S2: preparing a phosphorus source: preparing a phosphorus source solution, adding an alkali regulator to adjust the pH value to 8-8.5, and mixing with hydrogen peroxide to obtain a mixed solution. The phosphorus source is: disodium hydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid; the alkali regulator is: ammonia water, diammonium hydrogen phosphate or diammonium dihydrogen phosphate.

[0034] S3: preparing crude iron phosphate: passing the above mixed solution into the above ferrous chloride solution at 5-25° C. and stirring at a stirring speed of 300-400 rpm to obtain crude iron phosphate.

[0035] S4: Crystallization and aging: Add hydrochloric acid to the above crude ferric phosphate and adjust the pH to 0.8-1, and age at 92-95° C. to obtain a ferric phosphate solution.

[0036] S5: Preparation of iron phosphate: Filter the above iron phosphate solution using a vacuum filter or a filter press, wash the filtrate with ultrapure water until the conductivity is 200 μs / cm, then dry at 70-120°C, and calcine at 300-700°C to obtain battery-grade iron phosphate.

[0037] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to specific embodiments.

[0038] Example

[0039] Examples 1 and 2 of the present invention are implemented according to the technical scheme in the above-mentioned embodiment, and the particle size and yield of the iron phosphate particles and the impurity content in the battery-grade iron phosphate product are analyzed and tested. The specific implementation conditions and test results are shown in Table 1.

[0040] Table 1 Specific implementation and test results of Examples 1 and 2

[0041]

[0042] It can be seen from the test results in Table 1 that the technical solution of the present application can retain more ferrous ions in the ferrous chloride solution without loss, and effectively prepare the crude ferrous chloride byproduct of titanium dioxide into ferric phosphate, with good particle size and high yield. The impurity content of the battery-grade ferric phosphate product obtained after sintering has reached the standard for battery-grade lithium iron phosphate precursor, wherein the content of manganese is only about 13ppm, the iron-phosphorus ratio is stable at 0.96-0.975, and the physical phase is pure. The battery-grade ferric phosphate product of Example 1 was scanned by electron microscope, and the SEM image was obtained as follows: Figure 1 As shown, it can be seen that the iron phosphate element obtained in the present application has a uniform particle size distribution and can be preferably used as a precursor material.

[0043] Comparative Example 1

[0044] Comparative Example 1 uses the implementation method of the Chinese patent with application number CN202211623008.4 for comparative experiments. The difference from the embodiment is that the crude ferrous chloride is hydrolyzed and impurized by using an alkali regulator, and the pH value of the solution in the process is not strictly controlled. The yield of the obtained ferric phosphate particles is only 57%.

[0045] The embodiments described above are part of the embodiments of the present invention, rather than all of the embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the application claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

Claims

1. A method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, characterized in that: The following steps are involved: S1: crystal impurity removal: dissolving the crude ferrous chloride product and adjusting the pH value to 4-4.3 to obtain a ferrous chloride solution, wherein the content of ferrous chloride in the crude ferrous chloride product is 254-320 g / L; S2: preparing a phosphorus source: preparing a phosphorus source solution, adjusting the pH value of the phosphorus source to 8-8.5, and mixing the solution with hydrogen peroxide to obtain a mixed solution; S3: preparing crude ferric phosphate: passing the mixed solution into the ferrous chloride solution and stirring to obtain crude ferric phosphate; S4: crystallization and aging: adding hydrochloric acid to the crude ferric phosphate product and adjusting the pH to 0.8-1, aging at 92-95° C. to obtain a ferric phosphate solution; S5: preparing iron phosphate: filtering, washing, drying and calcining the iron phosphate solution to obtain battery-grade iron phosphate.

2. The method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, according to claim 1, characterized in that: The content of ferrous ions in the ferrous chloride solution is 112-140 g / L.

3. The method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, according to claim 1, characterized in that: In S2, an alkali regulator is used to adjust the pH value of the phosphorus source solution. The alkali regulator includes: ammonia water, diammonium hydrogen phosphate or ammonium dihydrogen phosphate.

4. A method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, according to claim 1 or 3, characterized in that: The phosphorus source includes: disodium hydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid.

5. The method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, according to claim 1, characterized in that: The S3 is carried out at 5-25° C., and the stirring speed is 300-400 rpm.

6. The method for preparing battery-grade ferric phosphate using ferrous chloride, a byproduct of titanium dioxide, according to claim 1, characterized in that: The filtration is performed using a vacuum filter or a filter press, and the washing includes: washing with ultrapure water to reduce the conductivity to 200 μs / cm, drying at 70 to 120° C., and calcining at 300 to 700° C.

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