Method for reducing content of magnesium, manganese and zinc impurity elements in iron phosphate

Abstract

The invention provides a method for reducing the content of magnesium, manganese and zinc impurity elements in iron phosphate, which comprises the following steps: (1) preparing a 1.2 mol / L ferrous sulfate solution, adding 25% ammonia water into the ferrous sulfate solution, adjusting the pH value, heating, stirring and reacting for 30 minutes, and filtering to obtain a purified ferrous sulfate solution; (2) preparing a 1 mol / L ammonium dihydrogen phosphate solution, and filtering to obtain a purified phosphorus salt solution; (3) dropwise adding 25% hydrogen peroxide into the purified ferrous sulfate solution to obtain a ferric sulfate solution; (4) dropwise adding a purified phosphorus salt solution into the oxidized ferric sulfate solution; (5) after the phosphorus salt solution is dropwise added, supplementing the purified ferrous sulfate solution according to the amount; (6) rapidly heating the solution and continuously reacting until the material turns white from yellow; and (7) washing the iron phosphate dihydrate precipitate until the conductivity is less than or equal to 200 us / cm, carrying out flash evaporation drying on the iron phosphate dihydrate filter cake, and calcining to obtain the battery-grade iron phosphate. The method can be widely applied to industrial production of iron phosphate.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of lithium ion battery positive electrode material precursor iron phosphate, and in particular to a method for reducing the content of magnesium, manganese and zinc impurity elements in iron phosphate. Background Art

[0002] In recent years, with the development of the new energy industry, lithium-ion batteries have attracted widespread attention due to their advantages of being green, environmentally friendly, having large specific capacity, and having a long cycle life. For lithium-ion battery cathode materials, iron phosphate precursors have also attracted great attention from scientists due to their controllable morphology and particle size for lithium iron phosphate and simple synthesis process, and have set off a wave of research on using iron phosphate as a precursor to synthesize lithium iron phosphate cathode materials.

[0003] Currently, the synthesis of iron phosphate is divided into a one-step method and a two-step method. The product of the two-step method has low content of impurity elements of magnesium, manganese and zinc, but its iron-phosphorus ratio has large uncontrollable fluctuations; the iron-phosphorus ratio of the one-step method product has a small controllable fluctuation range, but its impurity element content of magnesium, manganese and zinc is relatively high. How to obtain a production method with a stable and controllable iron-phosphorus ratio and low content of impurity elements of magnesium, manganese and zinc is the current production difficulty. Summary of the invention

[0004] The present invention provides a method for reducing the content of magnesium, manganese and zinc impurity elements in ferric phosphate, which can obtain the content of magnesium, manganese and zinc impurity elements comparable to the product of the two-step method while maintaining the controllable iron-phosphorus ratio of the one-step method. The present invention is achieved by the following technical solutions: A method for reducing the content of magnesium, manganese and zinc impurity elements in ferric phosphate comprises the following steps: (1) Prepare a 1.2 mol / L ferrous sulfate solution, then add 25% ammonia water to the ferrous sulfate solution, adjust its pH, heat and stir to react for 30 min, and filter to obtain a purified ferrous sulfate solution; (2) Prepare 1 mol / L diammonium dihydrogen phosphate solution and filter to obtain a purified phosphate solution; (3) adding 25% hydrogen peroxide to the purified ferrous sulfate solution to obtain a ferric sulfate solution; (4) adding dropwise the purified phosphate solution obtained in step (2) to the ferric sulfate solution oxidized in step (3); (5) After the phosphate solution has been added, add purified ferrous sulfate solution according to the amount; (6) Rapidly heating the above solution and continuing the reaction, and the material changes from yellow to white; (7) Washing the ferric phosphate dihydrate precipitate until the conductivity is ≤200 us / cm, flash drying the ferric phosphate dihydrate filter cake, and calcining to obtain battery-grade iron phosphate.

[0005] In the step (1), the impurity removal reaction temperature is 55±2°C, the reaction time is 30 min, and the pH at the end point of adding ammonia water is 4.2-4.5. The purification and impurity removal principle is through the increase of pH, wherein Fe 3+ 、Al 3+ 、Ti 4+ Becomes precipitated, reaction principle: Fe 3+ +OH - =Fe(OH)3 、Al 3+ +OH - =Al(OH)3 、TiOSO4+2H2O =TiO2﹒ H2O +H2SO4, filter accuracy is 1μm; The filtration accuracy of the filter in step (2) is 1 μm; In step (3), the hydrogen peroxide addition time is 60-90 min and the oxidation reaction temperature is 43-47°C; In step (4), the phosphorus salt is added dropwise for 60-90 minutes and the reaction temperature is controlled at 64-66°C; In step (5), 1% to 4% of the amount of ferrous iron in step (4) is added with purified ferrous sulfate, and the addition time is 9 to 10 minutes; In step (6), after the addition of purified ferrous sulfate is completed, the heating time is 30-40 min, the reaction temperature is 89-91° C., and the material is kept warm for 30-35 min after it turns from yellow to white; In step (7), the drying temperature is 60-80°C, the calcination process parameters are: heating rate 5-15°C / min, holding temperature 550-800°C, calcination time 4-6 h, and the material is cooled to 25-100°C; The beneficial effects of the present invention are: The present invention adds purified ferrous phosphate solution in the synthesis stage, greatly reduces the content of magnesium, manganese and zinc impurity elements in iron phosphate while maintaining a controllable iron-phosphorus ratio in the one-step method, and the iron phosphate material prepared by the method meets industry standards in terms of purity, particle size, morphology, iron-phosphorus ratio, etc., and the lithium iron phosphate positive electrode material prepared by the method has excellent electrochemical performance. The method can be widely applied to the industrial production of iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a scanning electron microscope picture of iron phosphate prepared in Example 1 of the present invention; Figure 2 is the XRD image of iron phosphate produced in Example 1 of the present invention; Figure 3 This is a graph showing the electrochemical performance of the lithium iron phosphate positive electrode material prepared in Example 1 of the present invention; DETAILED DESCRIPTION

[0007] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but the present invention is not limited in any way. Any changes or substitutions made based on the teachings of the present invention belong to the protection scope of the present invention.

[0008] Example 1 Step 1, prepare 1.2 mol / L ferrous sulfate solution, add 25% ammonia water, adjust its pH value to 4.3, heat to 55°C, react for 30 minutes, filter with a plate and frame filter press, and then fine filter with a bag filter with a filtration accuracy of 1 μm; Step 2, preparing 1.0 mol / L diammonium hydrogen phosphate solution, filtering it with a plate and frame filter press, and then fine filtering it with a bag filter with a filtration accuracy of 1 μm to obtain a purified phosphate solution; Step 3, adding 25% hydrogen peroxide to the purified ferrous sulfate solution, the dropping time is 60 min, and the oxidation temperature is 45° C. to obtain a ferric sulfate solution; Step 4, add the purified phosphate solution dropwise to the oxidized ferric sulfate solution, the amount of phosphate added is measured according to the iron-phosphorus ratio of the ferric solution of 0.945 (molar ratio), the phosphate addition time is 60 min, and the reaction temperature is 65°C; Step 5: After the phosphate salt is added, the temperature is raised to 90°C within 30 minutes, and the material is kept warm for 30 minutes after it turns from yellow to white; Step 6: Wash the ferric phosphate dihydrate with water using a vacuum filter press until its conductivity is ≤200 us / cm, flash dry the filter cake, and calcine at 550° C. to obtain battery-grade iron phosphate; Example 2 Step 1, prepare 1.2 mol / L ferrous sulfate solution, add 25% ammonia water, adjust its pH value to 4.3, heat to 55°C, react for 30 minutes, filter with a plate and frame filter press, and then fine filter with a bag filter with a filtration accuracy of 1 μm; Step 2, prepare 1.0 mol / L diammonium hydrogen phosphate solution, filter it with a plate and frame filter press, and then perform fine filtration with a bag filter with a filtration accuracy of 1 μm; Step 3, adding 25% hydrogen peroxide to the purified ferrous sulfate solution, the adding time is 60 min, and the oxidation temperature is 45°C; Step 4, add the purified phosphate solution dropwise to the oxidized ferric solution, the amount of phosphate added is measured according to the ferric solution iron-phosphorus ratio of 0.945 (molar ratio and iron content of the added ferrous solution also need to be considered), the phosphate addition time is 60 min, and the reaction temperature is 65°C; Step 5: After the addition of the phosphate salt is completed, add 2% of the ferrous solution (based on the amount of ferrous solution in step 3) for 10 minutes; Step 6: After the phosphorus salt is added, the temperature is raised to 90°C within 30 minutes, and the material is kept warm for 30 minutes after it turns from yellow to white; Step 7: washing the ferric phosphate dihydrate with water using a vacuum filter press until its conductivity is ≤200 us / cm, flash drying the filter cake, and calcining at 550° C. to obtain battery-grade iron phosphate; Example 3 Step 1, prepare 1.2 mol / L ferrous sulfate solution, add 25% ammonia water, adjust its pH value to 4.3, heat to 55°C, react for 30 minutes, filter with a plate and frame filter press, and then fine filter with a bag filter with a filtration accuracy of 1 μm; Step 2, prepare 1.0 mol / L diammonium hydrogen phosphate solution, filter it with a plate and frame filter press, and then perform fine filtration with a bag filter with a filtration accuracy of 1 μm; Step 3, adding 25% hydrogen peroxide to the purified ferrous sulfate solution, the adding time is 60 min, and the oxidation temperature is 45°C; Step 4, add the purified phosphate solution dropwise to the oxidized ferric solution, the amount of phosphate added is measured according to the ferric solution iron-phosphorus ratio of 0.945 (molar ratio and iron content of the added ferrous solution also need to be considered), the phosphate addition time is 60 min, and the reaction temperature is 65°C; Step 5: After the addition of the phosphate salt is completed, add 4% ferrous solution (based on the amount of ferrous solution in step 3) for 10 minutes; Step 6: After the addition of the additional phosphate salt is completed, the temperature is raised to 90°C within 30 minutes, and the material is kept warm for 30 minutes after it turns from yellow to white; Step 7: Wash the ferric phosphate dihydrate with water using a vacuum filter press until its conductivity is ≤200 us / cm, flash dry the filter cake, and calcine at 550° C. to obtain battery-grade iron phosphate.

Claims

1. A method for reducing the content of magnesium, manganese and zinc impurity elements in ferric phosphate, characterized in that The steps include: (1) Prepare a 1.2 mol / L ferrous sulfate solution, then add 25% ammonia water to the ferrous sulfate solution, adjust its pH, heat and stir to react for 30 min, and filter to obtain a purified ferrous sulfate solution; (2) Prepare 1 mol / L diammonium dihydrogen phosphate solution and filter to obtain a purified phosphate solution; (3) adding 25% hydrogen peroxide to the purified ferrous sulfate solution to obtain a ferric sulfate solution; (4) adding the purified phosphate solution dropwise to the oxidized ferric sulfate solution; (5) After the phosphate solution has been added, add purified ferrous sulfate solution according to the amount; (6) Rapidly heating the above solution and continuing the reaction, and the material changes from yellow to white; (7) Washing the ferric phosphate dihydrate precipitate until the conductivity is ≤200 us / cm, flash drying the ferric phosphate dihydrate filter cake, and calcining to obtain battery-grade iron phosphate.

2. The method according to claim 1, characterized in that: In the step (1), the impurity removal reaction temperature is 55±2°C, the reaction time is 30 min, and the pH at the end of the addition of ammonia water is 4.2-4.

5. The purification and impurity removal principle is through the increase of pH, wherein Fe 3+ 、Al 3+ 、Ti 4+ Becomes precipitated, reaction principle: Fe 3+ +OH - =Fe(OH)3 、Al 3+ +OH - =Al (OH)3 、TiOSO4+2H20 =TiO2.H2O +H2SO4, filter accuracy is 1μm.

3. The method according to claim 1, characterized in that: The filtering accuracy of the filter in step (2) is 1 μm.

4. The method according to claim 1, characterized in that: In the step (3), the hydrogen peroxide addition time is 60-90 minutes, and the oxidation reaction temperature is 43-47°C.

5. The method according to claim 1, characterized in that: In step (4), the phosphate salt is added for 60-90 minutes and the reaction temperature is controlled at 64-66°C.

6. The method according to claim 1, characterized in that: The amount of ferrous iron added in step (5) is 1%-4% of the amount of ferrous iron in step (4), and the addition time is 9-10 min.

7. The method according to claim 1, characterized in that: In step (6), after the addition of ferrous sulfate is completed, the heating time is 30-40 min, the reaction temperature is 89-91° C., and the material is kept warm for 30-35 min after it changes from yellow to white.

8. The method according to claim 1, characterized in that: In the step (7), the drying temperature is 60-80°C, the calcination process parameters are: heating rate 5-15°C / min, insulation temperature 550-800°C, calcination time 4-6 h, and the material is cooled to 25-100°C.

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