Production method of battery-grade iron phosphate based on raffinate acid and preparation method of lithium iron phosphate

Abstract

The invention relates to the technical field of iron phosphate production, and discloses a battery-grade iron phosphate production method based on raffinate acid and a preparation method of lithium iron phosphate thereof, and the method comprises the following steps: 1) adjusting the pH value of the raffinate acid with ammonia water, adding a fluorination auxiliary agent, carrying out heat preservation stirring, and filtering to obtain a filtrate A; (2) taking the preheated and impurity-removed ferrous sulfate solution as filtrate B, adding a mixture of the filtrate A and hydrogen peroxide into the filtrate B, regulating the pH value of the system by using ammonia water, preserving heat for at least 30 minutes, and filtering to obtain a primary filter cake; 3) washing the primary filter cake until the conductivity of the washing water is less than 3.5 ms / cm, mixing phosphoric acid, pure water and the filter cake for size mixing, aging until the color of the size is changed, continuously keeping the temperature, filtering, washing the secondary filter cake until the conductivity of the washing water is less than 2.5 ms / cm, drying, calcining and crushing to obtain battery-grade iron phosphate; the raffinate acid is used for replacing monoammonium phosphate or diammonium phosphate, the preparation process is shortened, the production efficiency is improved, and the material meeting the requirements of battery-grade iron phosphate can be directly prepared.

Description

Technical Field

[0001] The present invention relates to the technical field of iron phosphate production, and in particular to a method for producing battery-grade iron phosphate based on raffinate acid and a method for preparing lithium iron phosphate thereof. Background Art

[0002] The rapid development of the new energy lithium battery industry has led to an explosive growth in the demand for lithium iron phosphate materials, with production capacity increased to 2 million tons, and the production capacity of new entrants is still being released, posing a risk of overcapacity. Among them, iron phosphate, as a precursor of lithium iron phosphate materials, has also received widespread attention, and has attracted related companies such as phosphorus chemical industry and titanium chemical industry to make layouts. The production capacity has exceeded the demand, and the price has dropped from 24,000 yuan / ton to 9,000 yuan / ton. With the continuous release of production capacity, the price still has room to fall.

[0003] At present, competition in the iron phosphate industry has entered a white-hot stage. The downstream application end will inevitably pay more attention to the price, performance, and batch stability of iron phosphate. Therefore, it is urgent to develop a lower-cost iron phosphate preparation process while ensuring product quality.

[0004] The existing preparation method for reducing the production cost of iron phosphate is mainly through reducing the cost of raw materials, such as: the method disclosed in patent CN202311505206.5 uses phosphate rock, phosphorus iron ore, phosphide slag, low-quality acid and agricultural ammonium with a phosphorus grade of less than 20% after acid dissolution as a phosphorus source; using pyrite, pyrite slag, waste iron block and other leachate after acid dissolution as an iron source, and preparing crude iron phosphate through mixing, oxidation and precipitation steps, which all use cheap, easily available or discarded phosphorus and iron sources. Compared with the existing mass production process, the cost of raw materials can be saved by at least 15-20% per ton. However, using materials with too high impurity content as raw materials, after the iron phosphate is generated in a complex environment with multiple impurities, the impurity elements will be wrapped in the iron phosphate product. When the purity of the product needs to be further improved, the difficulty of impurity removal is greatly increased, and the whole process of this method requires multiple precipitations and a large amount of washing water.

[0005] Patent CN202010308437.7 discloses a low-cost method for preparing iron phosphate, which uses ferric chloride and phosphoric acid as raw materials and vanadium chloride as an additive to prepare an iron phosphate product with a small specific surface area and a high tap density. Scrap iron scraps are mixed with recycled hydrochloric acid to prepare an iron chloride solution, which can achieve element recycling and reduce production costs. However, in this process, the impurity element vanadium is introduced, and the vanadium content in the product reaches more than 500ppm. The use of scrap iron scraps as raw materials will also lead to a high manganese content in the product, reaching more than 300ppm, which is difficult to meet the requirements of battery-grade iron phosphate.

[0006] The requirements for battery-grade iron phosphate are that the content of metal ions such as calcium, magnesium, sodium, potassium, and aluminum is limited to 50ppm, 50ppm, 100ppm, 100ppm, and 300ppm respectively. In order to meet this requirement, monoammonium phosphate and diammonium phosphate are often used as phosphorus sources to produce iron phosphate to ensure that the impurity content in the obtained product meets the requirements. However, monoammonium phosphate and diammonium phosphate are sold at high prices, which increases production costs.

[0007] Raffinate acid is the main byproduct of the solvent extraction purification process of wet phosphoric acid, and its production accounts for 40% to 45% of the mass of the raw phosphoric acid, with a large production volume. The impurity ions in raffinate acid are mainly iron, aluminum, magnesium, sulfate, fluorine, arsenic and heavy metal ions. At present, raffinate acid is mixed with wet phosphoric acid in a mass ratio of 1: (2-5) to produce monoammonium phosphate, diammonium phosphate and compound fertilizer. The fertilizer obtained by this method has low grade. Raffinate acid is enriched with various elements in the raw phosphate ore and the used reagents, and has the characteristics of high viscosity and many impurities. It cannot be directly used for the synthesis of battery-grade iron phosphate, so the raffinate acid needs to be treated.

[0008] In order to further utilize the raffinate acid, the existing method mainly focuses on removing impurities from the raffinate acid and reacting it with an iron source to obtain iron phosphate, such as the method disclosed in patent CN202380009641.0, by adding phosphate concentrate to filter and precipitate, then adding sodium carbonate and activated diatomaceous earth to stir and react, then adding sodium sulfide nonahydrate and activated carbon powder to filter and precipitate, adding hydrogen peroxide to the obtained filtrate and then adding barium carbonate to filter to obtain the acid solution, which is concentrated, filtered, and separated, and then neutralized with concentrated ammonia, stirred in a water bath, and then washed and filtered with sodium carbonate aqueous solution, and the purified phosphoric acid is added to the obtained purified liquid many times, and then distilled to obtain purified phosphoric acid and concentrated to obtain industrial phosphoric acid. This method obtains industrial phosphoric acid through the above impurity removal steps. The method focuses on recovering phosphoric acid, and improving the purity of phosphoric acid to industrial grade before it can be used to prepare battery-grade iron phosphate. It does not consider directly obtaining iron phosphate using raffinate acid as a phosphoric acid source.

[0009] Another example is the production process for preparing battery-grade iron phosphate by wet-process phosphoric acid and related acids disclosed in patent CN202410528476.6. The method is to obtain NH4H2PO4 solution as a raw material by reacting with ferrous sulfate to obtain iron phosphate after three-stage three-stage countercurrent extraction of 2-ethylhexyl phosphate 2-ethylhexyl ester of raffinate, stripping, concentrated defluorination, chemical defluorination, re-extraction and re-stripping. However, the method needs to extract raffinate acid repeatedly, and still focuses on the reaction after removing impurities from raffinate acid to obtain phosphoric acid. The operation process is cumbersome, and multiple extractions and stripping are not suitable for industrial large-scale production, resulting in increased production costs. At the same time, the iron phosphate obtained by the method has a high magnesium content, and the electrical properties of the obtained material are not disclosed in the document, and there is a possibility of affecting the electrical properties of the subsequent battery materials.

[0010] Another example is the method disclosed in patent CN202211514595.3. It utilizes the characteristics of high P2O5% content and low content of impurities such as Ca, Mg and Al in phosphorus-containing waste after lithium extraction from lithium iron phosphate waste batteries. The phosphoric acid purification raffinate and other acids are compounded and reacted with phosphorus-containing waste to obtain iron phosphate. However, the impurity content of the iron phosphate obtained by this method is high, which is difficult to meet the requirements of battery-grade production raw materials. Summary of the invention

[0011] In response to the above technical problems, the present application provides a method for producing battery-grade iron phosphate based on raffinate acid, using phosphate raffinate acid as a phosphorus source, pre-treating the raffinate acid to remove some impurities and then directly conducting an iron phosphate synthesis reaction, and separating the product and impurities by adjusting the pH value of the synthesis system to obtain a product that meets the requirements of battery-grade iron phosphate.

[0012] The scheme of the present invention is:

[0013] A method for producing battery-grade iron phosphate based on raffinate acid comprises the following steps:

[0014] 1) preheating the solution to 55-65°C with ammonia water to extract the residual acid, and adjusting the pH value to 3.0-3.5, adding a fluorination aid, stirring and filtering to obtain a filtrate A;

[0015] 2) Preheat the ferrous sulfate solution after impurity removal as filtrate B to 40-50°C, add a mixture of filtrate A and hydrogen peroxide to filtrate B, adjust the pH value of the system to 1.5-2.5 with ammonia water, keep warm for ≥30 minutes and then filter to obtain a primary filter cake;

[0016] 3) After washing the primary filter cake until the conductivity of the washing water is less than 3.5 ms / cm, phosphoric acid and pure water are mixed with the filter cake to prepare a slurry, and the temperature is raised to 90-95° C. After aging until the slurry changes color, the slurry is kept warm and filtered. After washing the secondary filter cake until the conductivity of the washing water is less than 2.5 ms / cm, the secondary filter cake is dried, calcined, and crushed to obtain battery-grade iron phosphate;

[0017] The P2O5 concentration in the raffinate acid is 20-50%, and the fluorination auxiliary agent is one of ammonium fluoride and sodium fluorosilicate.

[0018] The method only requires adding a fluorination auxiliary agent to the raffinate acid and adjusting the pH value to obtain the original phosphorus source that can be used for subsequent use. The method has a short processing time and high efficiency, and is particularly suitable for large-scale industrial production.

[0019] As a preferred technical solution, the added mass amount of the fluorination auxiliary agent in the step 1) is 1-2% of the mass of the raffinate acid.

[0020] As a preferred technical solution, the concentration of the ammonia water is 20-30%.

[0021] As a preferred technical solution, the raffinate is preheated to 60°C and then the pH value is adjusted to 3.0 with aqueous ammonia.

[0022] As a preferred technical solution, the aging temperature in step 3) is 92°C.

[0023] As a preferred technical solution, the P2O5 concentration of the raffinate is 27%; after adjusting the pH of the raffinate to 3.0, a fluorination aid is added; the fluorination aid is ammonium fluoride; in step 2), the pH value of the system is adjusted to 1.8 with ammonia water before synthesizing iron phosphate.

[0024] As a preferred technical solution, the impurity remover for the ferrous sulfate solution is ammonia water; the amount of the impurity remover added is 2% of the mass of the ferrous sulfate solution; and the ferrous sulfate solution is preheated to 60°C.

[0025] The present invention also discloses a method for preparing lithium iron phosphate, comprising the following steps:

[0026] The iron phosphate prepared above is mixed with glucose, lithium carbonate and an auxiliary agent, and then ball-milled, spray-dried, calcined and ground to prepare lithium iron phosphate; the calcination temperature is 780-830°C.

[0027] Advantages of the present invention:

[0028] 1) The method for producing battery-grade iron phosphate based on raffinate provided by the present invention uses raffinate to replace monoammonium phosphate or diammonium phosphate, shortens the preparation process, improves production efficiency, and can directly produce materials that meet the requirements of battery-grade iron phosphate. After treatment by this method, the high content of Mg in the raffinate can be 2+ 、Na + , Mn 2+ , K + 、Al 3+ The impurity concentration is reduced and battery-grade iron phosphate can be directly synthesized.

[0029] 2) The battery-grade iron phosphate production method based on raffinate provided by the present invention uses ammonia water to adjust the pH value of the raffinate to reduce Mg 2+ 、Na + , Mn 2+ , K + The impurities are removed by about 50% to effectively reduce the impurity coating ratio in the synthesis process of iron phosphate. Then, in order to further reduce the Al2O3 insoluble in water in the residual acid used, 3+ content, adding fluoride additives to achieve Al 3+ After adding Al 3+ The removal rate reaches more than 95%. The pH value is adjusted during the subsequent synthesis to further separate the remaining impurities into the mother liquor, and after washing the first filter cake, the S and Mg that have been coated in FePO4·2H2O can be2 + 、Na + , Mn 2+ The impurities are further washed out to effectively avoid further encapsulation of impurities during the high-temperature aging and crystallization stage, and the aged slurry is filtered to obtain a secondary filter cake, which is washed again to further reduce the content of impurity ions in the obtained iron phosphate, thereby producing battery-grade iron phosphate.

[0030] 3) The battery-grade iron phosphate production method based on raffinate provided by the present invention, in which the iron phosphate completes the nucleation stage at a relatively low pH value, and the concentrations of iron ions and phosphate ions are relatively high, making it easier for the two to combine and form iron phosphate crystal nuclei, but since the reaction system at this time has restrictions on the growth of the crystal nuclei, the size of the crystal nuclei formed is relatively small. As the pH value of the reaction system increases, the precipitation reaction rate gradually slows down, but the crystal nuclei begin to grow further. Under a slightly higher pH environment, although the concentrations of iron ions and phosphate ions are reduced, there are still enough ions to supply the growth of the crystal nuclei, so that the particle size gradually increases, and in this process, the proportion of entering the iron phosphate can be effectively reduced, and the purity of the obtained iron phosphate can be effectively reduced.

[0031] 4) The battery-grade ferric phosphate production method based on raffinate provided by the present invention adopts a low pH system for nucleation and a high pH system for nucleus growth, which can effectively alleviate the agglomeration phenomenon between ferric phosphate particles during the production process. In a low pH environment, due to the fast nucleus formation speed and small particle size, it is difficult for the particles to agglomerate. Subsequently, a slightly higher pH environment is adopted. Although the particle size grows, the concentration of the reaction substance decreases, which increases the distance between the particles, effectively alleviating the agglomeration problem, thereby reducing the problem of excessive impurity content in the product caused by the coating of other impurities in the reaction system.

[0032] Compared with the nucleation reaction at a reaction system pH of about 2.0, nucleation at low pH can effectively avoid the encapsulation of impurities, and the overall particle size is smaller and more uniform. After raising the pH value of the reaction system to 2.0 with ammonia water, the fine particles grow evenly and the crystal form is more complete.

[0033] 5) The preparation method of lithium iron phosphate provided by the present invention has uniform iron phosphate particles and complete crystal form. This type of iron phosphate exhibits better high-temperature sintering resistance in the process of preparing lithium iron phosphate. At the same sintering temperature, the primary particles of the residual acid iron phosphate are smaller after sintering, and the number of large particles that break through the carbon layer is small. The sintering temperature can be increased by 30°C. The crystal integrity of the small particles is higher and the internal structure is denser, which is conducive to the preparation of higher-density lithium iron phosphate products. After the iron phosphate obtained by the above method is made into lithium iron phosphate, the compaction density is 2.595g / cm 3 ; 0.1C first charge 160.81mAh·g -1 ; 0.1C first discharge 158.43mAh·g-1 ; 0.1C initial efficiency 98.5%. DETAILED DESCRIPTION

[0034] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below in conjunction with specific embodiments.

[0035] Embodiment 1:

[0036] Take 1 kg of raffinate with a P2O5% concentration of 27%, preheat to 60°C, add ammonia water to adjust the pH to 3.0, add 15 g of solid ammonium fluoride, keep warm and stir for 1 hour, filter, and obtain filtrate A.

[0037] Take ferrous sulfate solution and add ammonia water with a concentration of 25% as an impurity remover. After adding 2% of the mass of ferrous sulfate solution, mix and stir to remove impurities, filter, take 1100g of the obtained filtrate, preheat to 60°C, add a mixture of filtrate A and hydrogen peroxide, and the feeding time is 30min. After the feeding is completed, use ammonia water to adjust the solution pH to 1.8, keep warm for 30min, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 3.5ms / cm, add appropriate amount of phosphoric acid and pure water to the filter cake after washing, heat to 92°C for aging, keep warm for 1h after the slurry changes color, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 2.5ms / cm, dry, calcine and crush after washing to obtain a battery-grade iron phosphate product.

[0038] Embodiment 2:

[0039] The remaining operations not described were carried out according to Example 1.

[0040] Take 1 kg of raffinate acid with a P2O5% concentration of 27%, preheat to 60°C, add ammonia water to adjust the pH to 2.5, add 15 g of solid ammonium fluoride, keep warm and stir for 1 hour, filter, and obtain filtrate A.

[0041] Take the ferrous sulfate solution, add the impurity remover to remove impurities, filter, take 1100g of the obtained filtrate, preheat to 40-50°C, add the mixture of filtrate A and hydrogen peroxide, the feeding time is 30min, after the feeding is completed, use ammonia water to adjust the solution pH to 1.8, keep warm for 30min, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 3.5ms / cm, after washing, add appropriate amount of phosphoric acid and pure water to the filter cake for slurry preparation, heat to 92°C for aging, keep warm for a period of time after the slurry changes color, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 2.5ms / cm, dry, calcine and crush after washing to obtain a battery-grade iron phosphate product.

[0042] Embodiment 3:

[0043] The remaining operations not described were carried out according to Example 1.

[0044] Take 1 kg of raffinate with a P2O5% concentration of 27%, preheat to 60°C, add ammonia water to adjust the pH to 3.0, add 15 g of solid ammonium fluoride, keep warm and stir for 1 hour, filter, and obtain filtrate A.

[0045] Take the ferrous sulfate solution, add an impurity remover to remove impurities, filter, take 1100g of the obtained filtrate, preheat and add a mixture of filtrate A and hydrogen peroxide, the feeding time is 30min, after the feeding is completed, use ammonia water to adjust the solution pH to 2.5, keep warm for 30min, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 3.5ms / cm, after washing, add appropriate amount of phosphoric acid and pure water to the filter cake for slurry preparation, heat to 92°C for aging, keep warm for a period of time after the slurry changes color, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 2.5ms / cm, dry, calcine and crush after washing to obtain a battery-grade iron phosphate product.

[0046] Embodiment 4:

[0047] The remaining operations not described were carried out according to Example 1.

[0048] Take 1 kg of raffinate acid with a P2O5% concentration of 27%, preheat to 60°C, add ammonia water to adjust the pH to 3.0, add 15 g of solid sodium fluorosilicate, keep warm and stir for 1 hour, filter, and obtain filtrate A.

[0049] Take the ferrous sulfate solution, add the impurity remover to remove impurities, filter, take 1100g of the obtained filtrate, preheat and add the mixture of filtrate A and hydrogen peroxide, the feeding time is 30min, after the feeding is completed, use ammonia water to adjust the solution pH to 1.8, keep warm for 30min, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 3.5ms / cm, after washing, add appropriate amount of phosphoric acid and pure water to the filter cake for slurry preparation, heat to 92°C for aging, keep warm for a period of time after the slurry changes color, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 2.5ms / cm, dry, calcine and crush after washing to obtain a battery-grade iron phosphate product.

[0050] Embodiment 5:

[0051] The remaining operations not described were carried out according to Example 1.

[0052] Take 1 kg of raffinate acid with a P2O5% concentration of 40%, preheat to 60°C, add ammonia water to adjust the pH to 3.0, add 15 g of solid sodium fluorosilicate, keep warm and stir for 1 hour, filter, and obtain filtrate A.

[0053] Take the ferrous sulfate solution, add the impurity remover to remove impurities, filter, take 1100g of the obtained filtrate, preheat and add the mixture of filtrate A and hydrogen peroxide, the feeding time is 30min, after the feeding is completed, use ammonia water to adjust the solution pH to 1.8, keep warm for 30min, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 3.5ms / cm, after washing, add appropriate amount of phosphoric acid and pure water to the filter cake for slurry preparation, heat to 92°C for aging, keep warm for a period of time after the slurry changes color, filter, wash the filter cake with pure water until the conductivity of the washing water is less than 2.5ms / cm, dry, calcine and crush after washing to obtain a battery-grade iron phosphate product.

[0054] Comparative Example 1:

[0055] The difference from Example 1 is:

[0056] Weigh 500 g of battery-grade monoammonium phosphate and substitute the raffinate acid to prepare solution A.

[0057] The iron phosphate products obtained in Examples 1 to 5 and Comparative Example 1 were tested for the following indicators. The testing methods were performed according to the commonly used methods in the art and are not described here. The results are shown in Table 1:

[0058] Table 1 Results of the ferric phosphate products obtained in Examples 1 to 5 and Comparative Example 1

[0059]

[0060]

[0061] The iron phosphate products obtained in Examples 1 to 5 and Comparative Example 1 were mixed with glucose, lithium carbonate and an auxiliary agent, and then ball milled, spray dried, calcined and ground to prepare lithium iron phosphate. Each operation process was carried out according to the commonly used operations and parameters in the art, and will not be repeated here. The various properties of the obtained lithium iron phosphate were tested according to the commonly used methods in the art, and the results are shown in Table 2:

[0062] Table 2 Performance index results of lithium iron phosphate obtained from iron phosphate obtained in Examples 1 to 5 and Comparative Example 1

[0063]

[0064] Examples 1 to 5 compare the effects of different raffinate acid impurity removal pH values, ferric phosphate synthesis pH values, impurity removal aids and raffinate acid concentrations on the physical and chemical indicators of ferric phosphate products.

[0065] As shown in Tables 1 and 2, when the raffinate acid with a P2O5 concentration of 27% is used, the raffinate acid pH is adjusted to 3.0, the impurity removal agent is ammonium fluoride, and the iron phosphate synthesis pH is 1.8, the physical and chemical indicators of the iron phosphate product can meet the battery grade requirements, the impurity content is low, and the compaction density of the lithium iron phosphate prepared using the iron phosphate reaches 2.595g / cm3 The first charge capacity at 0.1C reached 160.81mAh g -1 The first discharge capacity at 0.1C reached 158.43 mAh g -1 , the electrical performance is better.

[0066] When the pH value of the raffinate acid impurity removal is 2.5 (Example 3), impurities cannot be completely precipitated, the impurity content in the phosphate is relatively high, the overall impurity content of the prepared iron phosphate is relatively high, and the contents of Al, Cr, Ni, Zn, Mg, etc. are difficult to meet the requirements of battery-grade iron phosphate; when the pH value of the iron phosphate synthesis is increased to 2.5, more impurity ions are enriched from the mother liquor into the solid product, affecting the indicators of the iron phosphate product;

[0067] After the auxiliary agent ammonium fluoride was replaced with sodium fluorosilicate (Examples 4-5), the Al removal effect was similar, but the use of sodium fluorosilicate would introduce impurity ions, resulting in an increase in the amount of pure water used in the subsequent preparation process of ferric phosphate;

[0068] After increasing the raffinate concentration to 40% (Example 5), the impurity content of the iron phosphate product is less affected, but the compaction density of the prepared lithium iron phosphate material is low. During the synthesis of iron phosphate, a high concentration of phosphate salt is added to the system, resulting in a high local pH value. The product is easy to grow into larger particles, and some impurities are wrapped, resulting in a slightly higher impurity content in the iron phosphate product. The poor uniformity is not conducive to the subsequent grinding and calcination of lithium iron phosphate. The indicators of the obtained lithium iron phosphate are less than 27% of the raw material raffinate acid concentration.

[0069] In Comparative Example 1, battery-grade monoammonium phosphate is used as the phosphorus source for the synthesis of iron phosphate. Fewer impurities are introduced into the raw material end, and the obtained iron phosphate product has higher quality. The physical and chemical indicators are basically the same as those when the raffinate acid is used as the phosphorus source. The compacted density of lithium iron phosphate prepared using the iron phosphate in Comparative Example 1 is low, only 2.427 g / cm 3 The compaction density of iron phosphate prepared by residual acid can basically reach 2.55g / cm 3 After the two raw materials are finally made into lithium iron phosphate, the discharge capacity is 158 mAh g -1 It can be seen that when raffinate acid is used as the phosphorus source, the obtained lithium iron phosphate can achieve a higher compaction density, and the impurity content is similar to that of battery-grade monoammonium phosphate as the phosphorus source, indicating that the method provided by the present application uses raffinate acid with a higher impurity content and can also effectively reduce the impurity content in the final product.

[0070] After analysis and research, it was found that the use of fluoride as an auxiliary agent to remove aluminum ions during the removal of impurities from the residual acid results in a higher fluorine content in the iron phosphate prepared by this method. The presence of fluoride ions can form strong chemical bonds with iron ions, effectively increasing the melting point and thermal decomposition temperature of the crystals. Therefore, the calcination temperature can be appropriately increased to 780-830°C during the sintering process of lithium iron phosphate. Firstly, the crystallinity of lithium iron phosphate and the regularity of primary particles are improved. Secondly, due to its burning resistance, it can have a better uniform carbon layer coating at higher temperatures, which is beneficial to the improvement of its compaction density and the maintenance of better electrochemical properties.

[0071] Embodiment 6:

[0072] The difference from Example 1 is that the amount of raffinate added is 6.66 tons, and the amount of ferrous sulfate solution added is 7.333 tons. The various properties of the obtained iron phosphate are shown in Table 1; the various properties of lithium iron phosphate are shown in Table 2. It can be seen from Tables 1 and 2 that the method provided by this application can also achieve the same effect in the pilot stage and large-scale production.

[0073] Embodiment 7:

[0074] The difference from Example 1 is that the concentration of ammonia water in each step is 20%;

[0075] The P2O5 concentration in the raffinate used is 20%. After the raffinate is preheated to 65°C, the pH value is adjusted to 3.0 with aqueous ammonia. The amount of the added fluorination aid is 1% of the mass of the raffinate;

[0076] After the filtrate B was preheated to 40°C, when the pH value of the system was adjusted with ammonia water, the pH value of the system after adjustment was 1.5; the aging temperature was 90°C.

[0077] Embodiment 8:

[0078] The difference from Example 1 is that the concentration of ammonia water in each step is 30%;

[0079] The P2O5 concentration in the raffinate used is 50%; after the raffinate is preheated to 55°C, the pH value is adjusted to 3.5 with aqueous ammonia, and the amount of the added fluorination aid is 2% of the mass of the raffinate;

[0080] After the filtrate B was preheated to 50°C, when the pH value of the system was adjusted with ammonia water, the pH value of the system after adjustment was 2.5; the aging temperature was 95°C.

[0081] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and the description in the specification are only for illustrating the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of protection of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for producing battery-grade iron phosphate based on raffinate acid, characterized in that: The following steps are involved: 1) preheating the residual acid to 55-65°C with ammonia water to adjust the pH value to 3.0-3.5, adding a fluorination aid, stirring and filtering to obtain filtrate A; 2) Preheat the ferrous sulfate solution after impurity removal as filtrate B to 40-50°C, add a mixture of filtrate A and hydrogen peroxide to filtrate B, adjust the pH value of the system to 1.5-2.5 with ammonia water, keep warm for ≥30 minutes and then filter to obtain a primary filter cake; 3) After washing the primary filter cake until the conductivity of the washing water is less than 3.5 ms / cm, phosphoric acid and pure water are mixed with the filter cake to prepare a slurry, and the temperature is raised to 90-95° C. After aging until the slurry changes color, the slurry is kept warm and filtered. The secondary filter cake is washed until the conductivity of the washing water is less than 2.5 ms / cm, and then dried, calcined, and crushed to obtain battery-grade iron phosphate. The P2O5 concentration in the raffinate acid is 20-50%, and the fluorination auxiliary agent is one of ammonium fluoride and sodium fluorosilicate.

2. A method for producing battery-grade iron phosphate based on raffinate according to claim 1, characterized in that: In the step 1), the added mass amount of the fluorination aid is 1-2% of the mass of the raffinate acid.

3. A method for producing battery-grade iron phosphate based on raffinate as claimed in claim 1, characterized in that: The concentration of the ammonia water is 20-30%.

4. A method for producing battery-grade iron phosphate based on raffinate according to claim 1, characterized in that: The raffinate was preheated to 60°C and then the pH value was adjusted to 3.0 with aqueous ammonia.

5. A method for producing battery-grade iron phosphate based on raffinate as claimed in claim 1, characterized in that: The aging temperature in the step 3) is 92°C.

6. A method for producing battery-grade iron phosphate based on raffinate acid according to claim 1, characterized in that: The P2O5 concentration of the raffinate is 27%; after adjusting the pH value of the raffinate to 3.0, a fluorination aid is added; the fluorination aid is ammonium fluoride; in step 2), the pH value of the system is adjusted to 1.8 with ammonia water before synthesizing iron phosphate.

7. A method for producing battery-grade iron phosphate based on raffinate acid as claimed in claim 1, characterized in that: The impurity remover for the ferrous sulfate solution is ammonia water; the amount of the impurity remover added is 2% of the mass of the ferrous sulfate solution; the ferrous sulfate solution is preheated to 60°C.

8. A method for preparing lithium iron phosphate, characterized in that: The following steps are involved: The lithium iron phosphate is prepared by mixing the iron phosphate obtained by the method according to any one of claims 1 to 7 with glucose, lithium carbonate and an auxiliary agent, and then ball milling, spray drying, calcining and grinding; the calcination temperature is 780 to 830°C.

Images