A method for synthesizing iron phosphate with low impurity content by using ferrous sulfate, a by-product of titanium dioxide

Abstract

The present invention discloses a method for synthesizing iron phosphate with low impurity content by using ferrous sulfate, a by-product of titanium dioxide, which includes the steps of: successively adding an alkaline and an acidic impurity remover into a ferrous sulfate solution of the by-product of titanium dioxide, filtering after two impurity removal reactions to obtain a clear ferrous sulfate solution A; preparing a mixed phosphate solution B; adding the mixed solution B into the ferric salt solution A, filtering the slurry after the reaction, and washing to obtain a filter cake C; adding the filter cake C into a dilute phosphoric acid solution for slurry adjustment, and then performing high-temperature crystal transformation and aging to obtain a slurry D; filtering and washing the slurry D, repeating the filtering and washing operations to obtain a filter cake E; drying, high-temperature calcining, and crushing the filter cake E to obtain an anhydrous iron phosphate product; the iron phosphate product prepared by the present invention has low impurity content and stable quality, and can be used as an ideal precursor cathode material for preparing high-capacity lithium iron phosphate batteries.

Description

Technical Field

[0001] The present invention relates to the field of inorganic materials, and particularly to a method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide. Background Art

[0002] Benefiting from the rapid development of new energy vehicles and energy storage industries in China in recent years, lithium iron phosphate has gradually exceeded ternary materials and gained market recognition due to its high safety, long cycle life, low cost and other advantages. Its demand is increasing continuously, and the production capacity has increased from 181,200 tons / year at the end of 2018 to 898,000 tons / year at the end of 2021, with a compound annual growth rate of 70.5%. The year-on-year growth rate in 2021 was even as high as 167.9%. Among them, iron phosphate, as an important precursor for the production of lithium iron phosphate, its quality indicators such as impurity content, crystal structure and microscopic morphology have extremely important effects on the electrochemical performance of downstream lithium iron phosphate batteries. At present, iron phosphate is usually prepared by reacting an iron source with a phosphorus source. Most iron phosphate manufacturers in the industry usually use by-product ferrous sulfate of titanium dioxide as the iron source and phosphoric acid or phosphate as the phosphorus source to prepare iron phosphate for batteries in order to reduce production costs.

[0003] The main production methods of titanium dioxide are the sulfuric acid method and the chloride method. Among them, domestic titanium dioxide production enterprises mainly use the sulfuric acid method. In the process of producing iron dioxide by the sulfuric acid method, 3.5 - 4.0 tons of ferrous sulfate heptahydrate are by-produced for every 1 ton of titanium dioxide product. Estimated based on the output of sulfuric acid method titanium dioxide being 3 million tons in 2021, about 10 - 12 million tons of ferrous sulfate are by-produced throughout the year. Since the content of metal impurity elements, especially Ti, Mg, Al, and Mn, in the by-product ferrous sulfate of titanium dioxide is relatively high, it is difficult to be directly used to synthesize iron phosphate. A suitable pretreatment process needs to be adopted to remove impurities and purify it. At the same time, the process of controlling the impurity content during the synthesis process is the technical key to preparing high-purity and low-impurity-content ferrous sulfate from the by-product ferrous sulfate of titanium dioxide. Chinese patent application document CN106892415A discloses a method for preparing iron phosphate from the by-product ferrous sulfate of titanium dioxide, specifically disclosing that the impurity-removed ferrous sulfate solution is obtained by heating, blowing air, adding sulfuric acid to adjust the pH value, adding a flocculant, and then adding sodium dihydrogen phosphate and filtering. Finally, iron phosphate is prepared by adding sodium phosphate and a phosphoric acid conversion reaction. In this method, the impurity removal reaction temperature of ferrous sulfate is relatively high. After blowing air, ferrous ions are prone to hydrolyze and precipitate in large amounts, resulting in a large iron loss and a low product yield. Moreover, the impurity removal process in this process is complex, with strict requirements for the impurities in the raw material by-product ferrous sulfate of titanium dioxide. At the same time, the amount of washing water is large during the washing process of preparing iron phosphate, and it is difficult to control the quality of iron phosphate. Chinese patent application document CN107857243A discloses a method for preparing battery-grade ultrafine iron phosphate from by-product ferrous sulfate. After adding phosphoric acid to form iron phosphate colloid with divalent iron in the solution to adsorb some impurities and then adding a flocculant, after sedimentation and filtration, ammonia water is added to convert ferrous sulfate into iron hydroxide, and then phosphoric acid is added to convert it into iron phosphate. Although the colloid formed by adding phosphoric acid to adsorb impurities in this method has a good effect on the removal of Ti, it has a poor adsorption effect on Mn, Al, and Mn impurities. At the same time, during the synthesis process of iron phosphate, due to the addition of ammonia water, impurities are easily co-precipitated with iron and carried into iron hydroxide. After being converted into iron phosphate, the impurity elements Mg, Al, and Mn in the finished iron phosphate are relatively high. In addition, during the post-stage crystal conversion and aging reaction process of iron phosphate, the amount of phosphoric acid added will be affected by the phosphoric acid introduced in the impurity removal stage, resulting in inaccurate phosphorus source measurement, difficult control of Fe / P, causing fluctuations in the quality stability and consistency of the product, and having an adverse impact on the electrical performance of downstream products. Based on this, it is still necessary to further develop a process for synthesizing high-purity, low-impurity-content, and quality-stable iron phosphate using the by-product ferrous sulfate of titanium dioxide with simple operation and high controllability. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems in the current industry that during the synthesis of iron phosphate using by - product ferrous sulfate of titanium dioxide, due to the introduction of raw material impurities and a large amount of ammonium sulfate mother liquor generated during the reaction process of iron phosphate, a large amount of metal and sulfur impurities are coated on the product, and the fluctuation of the pH of the reaction system causes incomplete precipitation reaction of iron phosphate and the generation of metal miscellaneous salts, resulting in high impurity content of the product, unstable iron - phosphorus ratio, and poor quality stability between batches. A method for industrial preparation of iron phosphate for batteries with low impurity content and stable product quality between batches is provided.

[0005] To achieve the above - mentioned purpose, a method for synthesizing iron phosphate with low impurity content using by - product ferrous sulfate of titanium dioxide is as follows: First, using ferrous sulfate after deep impurity removal and purification treatment and industrial - grade phosphate after impurity removal and pH adjustment as raw materials, the iron phosphate intermediate slurry is synthesized by the liquid - phase precipitation method. After filtration and washing, it is re - slurried, and then subjected to high - temperature crystal transformation and aging to obtain the dihydrate iron phosphate slurry. Finally, through filtration, washing, drying, high - temperature calcination, and crushing, an anhydrous iron phosphate product with stable quality and low impurity content is obtained. The method mainly includes the following steps:

[0006] S1. Take the by - product ferrous sulfate of titanium dioxide, dissolve it in water to obtain a saturated solution of ferrous sulfate. While stirring, add the alkaline impurity remover to the saturated solution of ferrous sulfate, control the pH of the solution at 4.0 - 6.0 for impurity removal reaction. After the obtained slurry is filtered, the ferrous sulfate solution is obtained and then the acidic impurity remover is added for secondary impurity removal reaction, control the pH of the solution at 2.0 - 3.0. After the obtained slurry is filtered, the refined ferrous sulfate solution A is obtained, and it is diluted with water to a Fe 2+ concentration of 0.80 - 1.25 mol / L for standby;

[0007] S2. Dissolve the industrial - grade phosphate in water to prepare a phosphate solution with a concentration of 0.5 - 1.2 mol / L, add a pH regulator to adjust the pH of the solution to 6.0 - 8.0 for impurity removal reaction. The filtered phosphate clear liquid is added with a certain excess coefficient of hydrogen peroxide, and after stirring evenly, a phosphate mixed solution B is obtained, and the pH value of the mixed solution is controlled at 6.5 - 7.5;

[0008] S3. Rapidly add the phosphate solution B obtained in step S2 to the ferrous sulfate solution A obtained in step S1, heat up to 45 - 60 °C for reaction, control the pH of the synthesis reaction slurry at 2.5 ± 0.5. After reacting for 20 - 30 min, a light - yellow iron phosphate intermediate slurry C is obtained. At the same time, detect whether there is Fe 2+ in the slurry. If so, add hydrogen peroxide to the slurry until there is no Fe 2+ ;

[0009] S4. Filter the light - yellow iron phosphate intermediate suspension C obtained in step S3, and repeatedly wash it with pure water at 60 - 70 °C until the conductivity of the washing water ≤ 5 ms / cm to obtain the filter cake D;

[0010] S5. Add a certain amount of dilute phosphoric acid solution according to the solid content of 10.0 - 15.0% based on FePO4·2H2O to slurry the filter cake D obtained in step S3 to obtain a suspension E. The concentration of the dilute phosphoric acid solution is 2.0 - 4.0% based on H3PO4, and the ratio of total P to total Fe in the reaction system is controlled to be 1.05 - 1.30:1. After aging and crystal transformation reaction under high temperature and stirring conditions, a white iron phosphate slurry F is obtained.

[0011] S6. Filter the iron phosphate slurry F obtained in step S5, repeat washing until the conductivity of the washing water ≤ 2500 μs / cm. The filter cake obtained after washing is dried to remove surface water, calcined at high temperature to remove free water, and then crushed to obtain an anhydrous iron phosphate product. The washing water with conductivity ≤ 3500 during the washing process is mixed with concentrated phosphoric acid to prepare the dilute phosphoric acid solution required in step S5.

[0012] As a preferred technical solution, in step S1, the basic impurity remover is one of ammonia water, sodium hydroxide, potassium hydroxide or calcium hydroxide, and the acidic impurity remover is one of phosphoric acid and sulfuric acid.

[0013] As a further preference, in step S1, the basic impurity remover is ammonia water, and the acidic impurity remover is phosphoric acid.

[0014] As a preferred technical solution, the impurity removal reaction temperature is 50 - 60 °C, and the reaction time is 30 min - 90 min.

[0015] As a preferred technical solution, in step S2, the pH regulator is one of ammonia water, sodium hydroxide, and potassium hydroxide.

[0016] As a further preference, in step S2, the pH regulator is ammonia water.

[0017] As a preferred technical solution, the impurity removal reaction temperature is 40 - 50 °C, the reaction time is 20 min - 30 min, and the excess coefficient of hydrogen peroxide is 1.10 - 1.40.

[0018] As a preferred technical solution, in step S3, the rapid feeding time of the phosphate salt mixed solution B is 5 - 15 min, and the indicator for detecting whether there is Fe 2+ in the test slurry is bipyridine indicator.

[0019] As a preferred technical solution, in step S3, the addition amount of the phosphate salt solution B is added according to the molar ratio of phosphate salt to iron salt of 0.85 - 0.95:1.

[0020] Due to the adoption of the above technical solution, a method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide includes the following steps: (1) successively adding an alkaline and an acidic impurity remover into the by-product ferrous sulfate solution of titanium dioxide, carrying out two impurity removal reactions and then filtering to obtain a ferrous sulfate clear solution A with a lower impurity content; (2) taking industrial-grade phosphate, dissolving it in water to form a phosphate solution, adding a pH regulator to adjust the pH of the solution to 6.0 - 8.0 for impurity removal reaction, removing the main metal impurities affecting the quality of the iron phosphate product, adding hydrogen peroxide with a certain excess coefficient to the filtered phosphate mixed solution, and stirring evenly to obtain a phosphorus salt mixed solution B; (3) quickly adding the mixed solution B to the ferric salt solution A, filtering the slurry after reacting for a period of time, and washing to obtain a filter cake C; (4) adding the filter cake C to a dilute phosphoric acid solution for slurry adjustment, controlling the molar ratio of total P to total Fe in the reaction system to be 1.05 - 1.30, and then carrying out high-temperature crystal transformation and aging to obtain a slurry D; (5) filtering and washing the slurry D, repeating the filtering and washing operations until the conductivity of the washing water ≤ 2500 μs / cm to obtain a filter cake E, and taking the washing water with a conductivity ≤ 3500 during the washing process to prepare the dilute phosphoric acid solution used for slurry adjustment; (6) drying, high-temperature calcining, and crushing the filter cake E to obtain an anhydrous iron phosphate product. The iron phosphate product prepared by the present invention has low impurity content and stable quality, and can be used as an ideal precursor cathode material for preparing high-capacity lithium iron phosphate batteries.

[0021] Advantages of the present invention:

[0022] The present invention provides an industrial preparation method of iron phosphate for batteries with low impurity content and stable product quality between batches. First, using purified ferrous sulfate after impurity removal and phosphate after impurity removal and pH adjustment as raw materials, a liquid-phase precipitation method is adopted to first synthesize an iron phosphate intermediate slurry. After filtration and washing, it is re-slurried with dilute phosphoric acid, then high-temperature crystal transformation and aging are carried out to obtain a dihydrate iron phosphate slurry, and finally, through filtration, washing, drying, high-temperature calcining, and crushing, an anhydrous iron phosphate product with stable quality and low impurity content is obtained.

[0023] The present invention uses by-product ferrous sulfate of titanium dioxide as a raw material, first adds an alkaline impurity remover to remove Al, Zn, Mn, and Mg impurity elements in the raw material, then adds an acidic impurity remover to perform secondary removal of Ti, Mg, and Mn in the raw material. After two impurity removals, a ferrous source raw material with a lower impurity content is obtained. At the same time, by adding the alkaline impurity remover first and then the acidic impurity remover, the pH of the ferrous sulfate solution is appropriately adjusted back to prevent Fe under high pH conditions 2+Oxidation and formation of Fe(OH)3 to ensure the stability of ferrous raw materials; industrial-grade phosphate is used to replace the relatively expensive water-soluble and battery-grade phosphate, and through impurity removal reaction, the use cost of phosphorus source is reduced on the basis of ensuring that the raw materials meet the impurity content requirements; by increasing the concentration of iron salt and shortening the feeding time of phosphate mixed solution, the number of ferric phosphate crystal nuclei increases and the crystals nucleate rapidly during the synthesis reaction, reducing the coating of impurities during the growth of crystal nuclei caused by too long feeding time and reducing the amount of subsequent washing water; making full use of the characteristics of high phosphorus and low impurity content in the washing water of the second washing of crystal conversion and aging, realizing the recycling of washing water and valuable phosphorus elements, and reducing the treatment load of the water system in the later stage; using the characteristic that dilute phosphoric acid solution is easier to disperse the filter cake obtained by the first plate and frame more evenly during the repulping process, and advancing the addition point of phosphoric acid required for crystal conversion and aging from the crystal conversion and aging process to the repulping process to ensure the uniformity of the aged ferric phosphate slurry.

[0024] In summary, through the above innovative invention measures, the present invention overcomes the problems existing in the synthesis of ferric phosphate using by-product ferrous sulfate of titanium dioxide in the current industry, such as high impurity content of raw materials, large process fluctuations, unstable iron-phosphorus ratio, and poor quality stability between batches, and obtains ferric phosphate products with low impurity content and stable product quality between batches. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the process flow chart of the present invention;

[0026] Figure 2 is the SEM image of the ferric phosphate prepared in Example 1;

[0027] Figure 3 is the XRD pattern of the ferric phosphate prepared in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention provides a method for synthesizing ferric phosphate with low impurity content using by-product ferrous sulfate of titanium dioxide.

[0029] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0030] Example 1:

[0031] (1) Take ferrous sulfate, dissolve it in water to prepare a saturated solution, add 20% NH3·H2O to adjust the pH of the ferrous solution to 5.5, react for 90 min at 50 °C for impurity removal, and the obtained slurry is filtered by plate and frame to obtain a ferrous sulfate solution, then add 15% phosphoric acid (calculated as H3PO4) and react for 25 min at 45 °C. The obtained slurry is filtered by plate and frame to obtain a ferrous sulfate clear liquid, and diluted with water to Fe 2+Reserve it with a concentration of 0.85 mol / L;

[0032] (2) Take a phosphate solution with a concentration of 0.65 mol / L, add 20% NH₃·H₂O to adjust the pH of the solution to 6.8, react at 45 °C for 20 min, obtain a slurry, filter it through a plate-and-frame filter to get a clear phosphate solution, add 27% hydrogen peroxide according to 1.20 eq of the theoretical dosage for completely oxidizing ferrous ions to ferric ions, stir evenly to obtain a phosphate mixed solution, and the pH of the mixed solution is 6.5;

[0033] (3) Take the prepared ferrous sulfate solution, under stirring conditions, add the phosphate solution to the ferrous solution according to the addition amount with a molar ratio of phosphate to ferric salt of 0.87:1. The addition time of the phosphate mixed solution is 10 min. After the addition is completed, raise the temperature to 52 °C for reaction, control the pH of the synthetic reaction slurry to be 2.2 ± 0.2, and after reacting for 25 min, obtain a yellow iron phosphate intermediate suspension;

[0034] (4) Filter the obtained yellow iron phosphate intermediate suspension, wash it repeatedly with pure water at 65 °C and a conductivity ≤ 100 μs / cm until the conductivity of the washing liquid ≤ 5 ms / cm to obtain a filter cake. Add 3% dilute phosphoric acid solution according to a solid content of 12.0% (calculated as FePO₄·2H₂O), and slurry the washed filter cake to obtain a suspension;

[0035] (5) Add phosphoric acid to the suspension E, control the total P and total Fe molar ratio of the reaction system to be 1.10:1, quickly raise the temperature to 88 °C under the stirring speed of 200 r / min for high-temperature crystal transformation and aging reaction, and keep it warm for 90 min under this condition to obtain a white iron phosphate slurry;

[0036] (6) Filter the obtained white iron phosphate slurry, wash the filter cake with wash water with a conductivity ≤ 100 μs / cm until the conductivity of the wash water is 2500 μs / cm to obtain a white filter cake. Dry the filter cake at 105 °C for 3.5 h, then calcine it at 580 °C for 4.5 h under high temperature, and obtain an anhydrous iron phosphate product after crushing and grinding.

[0037] The quality analysis results of the anhydrous phosphoric acid product in this example are shown in Table 1. It can be seen from Table 1 that the Fe / P of the iron phosphate product synthesized by the present invention is 0.964, meeting the requirements of 0.965 ± 0.05 for mainstream market products; D50 < 4 μm, and the particle size distribution is uniform, meeting the requirements for the particle size of the product; the BET (specific surface area) distribution range of the product is relatively narrow, between 8 - 9 m 2 ² / g; the tapped density is 0.95 g / m 3 ³, far greater than the requirement of 0.60 g / m 3 ³ in the HG / T 4701 - 2004 standard for iron phosphate for batteries.According to the requirements, the impurity elements in the product are much lower than those in the iron phosphate products obtained by traditional processes. The SEM and XRD diagrams of the anhydrous iron phosphate product are as shown in Figure 2 and Figure 3 respectively. It can be seen from the SEM diagram of the product that the iron phosphate particles synthesized by the present invention have uniform particle size and are of porous structure. The lithium iron phosphate product synthesized from iron phosphate with this microscopic morphology has a high specific surface area, which increases the contact area between the electrolyte and the cathode material, is conducive to the diffusion of lithium ions, solves the disadvantages of poor conductivity and rate performance of micron-sized lithium iron phosphate particles, and at the same time retains a high tap density, increases the discharge capacity of the battery, reduces the internal resistance, reduces the polarization loss, achieves the purpose of extending the cycle life of the battery and improving the utilization rate of lithium ion batteries. It can be seen from the XRD diagram of the product that all the diffraction peak positions and intensities of the iron phosphate synthesized by the present invention correspond to the standard card PDF#84-0876, and there are no impurity peaks in the diffraction pattern, and the purity of the product is relatively high.

[0038] Table 1 Results of physical and chemical analysis and ICP comparative analysis of the product

[0039]

[0040]

[0041] Example 2:

[0042] (1) Take ferrous sulfate, dissolve it in water to prepare a saturated solution, add 20% NH3·H2O to adjust the pH of the ferrous solution to 6.0, react for 70 min at 55 °C for impurity removal, and filter the obtained slurry through a plate-and-frame filter to obtain a ferrous sulfate solution. Then add 15% phosphoric acid (calculated as H3PO4) and react for 30 min at 45 °C. Filter the obtained slurry through a plate-and-frame filter to obtain a clear ferrous sulfate solution, and dilute it with water to a Fe 2+ concentration of 0.90 mol / L for standby;

[0043] (2) Take a phosphate solution with a concentration of 0.60 mol / L, add 20% NH3.H2O to adjust the pH of the solution to 7.2, react for 30 min at 48 °C, filter the obtained slurry through a plate-and-frame filter to obtain a clear phosphate solution, add 27% hydrogen peroxide according to 1.18 eq of the theoretical amount for oxidizing all ferrous ions to ferric ions, stir evenly to obtain a phosphate mixed solution, and the pH of the mixed solution is 6.8;

[0044] (3) Take the prepared ferrous sulfate solution, and under stirring conditions, add the phosphate solution to the ferrous solution according to an addition amount with a molar ratio of phosphate to ferric salt of 0.90:1. The addition time of the phosphate mixed solution is 15 min. After the addition is completed, raise the temperature to 52 °C for reaction, control the pH of the synthetic reaction slurry to be 2.3 ± 0.2, and after reacting for 25 min, obtain a yellow iron phosphate intermediate suspension;

[0045] (4) Filter the obtained yellow iron phosphate intermediate suspension, repeatedly wash it with pure water at 60 °C and conductivity ≤ 100 μs / cm until the conductivity of the washing liquid is 4500 μs / cm to obtain a filter cake. Add a 2.5% dilute phosphoric acid solution according to a solid content of 11.0% (calculated as FePO4·2H2O), and pulp the washed filter cake to obtain a suspension.

[0046] (5) Add phosphoric acid to the suspension E, control the molar ratio of total P to total Fe in the reaction system to be 1.12:1, quickly heat up to 90 °C under stirring conditions for high-temperature crystal transformation and aging reaction, and keep it at this temperature for 70 min to obtain a white iron phosphate slurry.

[0047] (6) Filter the obtained white iron phosphate slurry, wash the filter cake with washing water with conductivity ≤ 100 μs / cm until the conductivity of the washing water is 1800 μs / cm to obtain a white filter cake. Dry the filter cake at 105 °C for 4.5 h, and then calcine it at 600 °C for 4.0 h. After crushing and grinding, an anhydrous iron phosphate product is obtained.

[0048] Example 3:

[0049] (1) Take ferrous sulfate, dissolve it in water to prepare a saturated solution, add 25% NH3·H2O to adjust the pH of the ferrous solution to 5.7, react for 60 min at 58 °C for impurity removal. The obtained slurry is filtered through a plate and frame filter to obtain a ferrous sulfate solution, and then add 20% phosphoric acid (calculated as H3PO4) and react for 20 min at 45 °C. The obtained slurry is filtered through a plate and frame filter to obtain a clear ferrous sulfate solution, and dilute it with water to a Fe 2+ concentration of 0.94 mol / L for standby.

[0050] (2) Take a phosphate solution with a concentration of 0.71 mol / L, add 20% NH3.H2O to adjust the pH of the solution to 7.0, react for 25 min at 45 °C. The obtained slurry is filtered through a plate and frame filter to obtain a clear phosphate solution. Add 27% hydrogen peroxide according to 1.25 eq of the theoretical dosage for completely oxidizing ferrous ions to ferric ions, stir evenly to obtain a phosphate mixed solution, and the pH of the mixed solution is 6.7.

[0051] (3) Take the prepared ferrous sulfate solution. Under stirring conditions, add the phosphate solution to the ferrous solution according to an addition amount with a molar ratio of phosphate to ferric salt of 0.88:1. The addition time of the phosphate mixed solution is 10 min. After the addition is completed, heat up to 58 °C for low-temperature reaction, control the pH of the synthetic reaction slurry to be 2.3 ± 0.2, and after reacting for 30 min, obtain a yellow iron phosphate intermediate suspension.

[0052] (4) Filter the obtained yellow iron phosphate intermediate suspension, and repeatedly wash it with pure water at 68°C and a conductivity ≤ 100 μs / cm until the conductivity of the washing liquid is 3200 μs / cm to obtain a filter cake. Add a 2.0% dilute phosphoric acid solution according to a solid content of 13.0% (calculated as FePO4·2H2O), and slurry the washed filter cake to obtain a suspension;

[0053] (5) Add phosphoric acid to the suspension E, control the molar ratio of total P to total Fe in the reaction system to 1.17:1, rapidly heat up to 87°C under stirring conditions for high-temperature crystal transformation and aging reaction, and keep it warm for 120 min under this condition to obtain a white iron phosphate slurry;

[0054] (6) Filter the obtained white iron phosphate slurry, wash the filter cake with washing water with a conductivity ≤ 100 μs / cm until the conductivity of the washing water is 1900 μs / cm to obtain a white filter cake. After drying the filter cake at 105°C for 4.0 h, calcine it at 620°C for 3.5 h under high temperature, and obtain an anhydrous iron phosphate product after crushing and grinding.

[0055] Example 4:

[0056] (1) Take ferrous sulfate, dissolve it in water to prepare a saturated solution, add 22% NH3·H2O to adjust the pH of the ferrous solution to 5.4, react for 45 min at 60°C for impurity removal. The obtained slurry is filtered through a plate and frame filter press to obtain a ferrous sulfate solution, and then add 15% phosphoric acid (calculated as H3PO4) and react for 20 min at 48°C. The obtained slurry is filtered through a plate and frame filter press to obtain a clear ferrous sulfate solution, and dilute it with water to a Fe 2+ concentration of 1.04 mol / L for standby;

[0057] (2) Take a phosphate solution with a concentration of 0.90 mol / L, add 20% NH3.H2O to adjust the pH of the solution to 7.2, react for 20 min at 50°C, filter the obtained slurry through a plate and frame filter press to obtain a clear phosphate solution, add 27% hydrogen peroxide according to 1.30 eq of the theoretical dosage for completely oxidizing ferrous ions to ferric ions, stir evenly to obtain a phosphate mixed solution, and the pH of the mixed solution is 6.8;

[0058] (3) Take the prepared ferrous sulfate solution, and add the phosphate solution to the ferrous solution at an addition amount with a molar ratio of phosphate to ferric salt of 0.92:1 under the condition of a stirring speed of 300 r / min. The addition time of the phosphate mixed solution is 7 min. After the addition is completed, heat up to 52°C for low-temperature reaction, control the pH of the synthetic reaction slurry to 2.4 ± 0.1, and after reacting for 25 min, obtain a yellow iron phosphate intermediate suspension;

[0059] (4) Filter the obtained yellow iron phosphate intermediate suspension, and repeatedly wash it with pure water at 60°C and a conductivity ≤ 100 μs / cm until the conductivity of the washing liquid is 1500 μs / cm to obtain a filter cake. Add a 3.2% dilute phosphoric acid solution according to a solid content of 14.0% (calculated as FePO₄·2H₂O), and slurry the washed filter cake to obtain a suspension;

[0060] (5) Add phosphoric acid to the suspension E, control the molar ratio of total P to total Fe in the reaction system to be 1.15:1, and quickly heat it to 92°C under a stirring speed of 200 r / min for a high-temperature crystal transformation and aging reaction. Keep it at this temperature for 60 min to obtain a white iron phosphate slurry;

[0061] (6) Filter the obtained white iron phosphate slurry, wash the filter cake with washing water with a conductivity ≤ 100 μs / cm until the conductivity of the washing water is 2100 μs / cm to obtain a white filter cake. Dry the filter cake at 105°C for 4.5 h, and then calcine it at 570°C for 4.5 h under high temperature. After crushing and grinding, an anhydrous iron phosphate product is obtained.

[0062] As can be seen from Table 2, the iron phosphate products produced by the present invention in an industrial device have good consistency between batches. The Fe / P deviation between batches of products is ±0.001, and indexes such as D50, specific surface area, and tapped density are stable. The impurity content is significantly lower than that of the traditional process. Among them, sodium (Na) ≤ 5 ppm, potassium (K) ≤ 5 ppm, calcium (Ca) ≤ 10 ppm, copper (Cu) ≤ 5 ppm, Zn ≤ 20 ppm, chromium (Cr) ≤ 10 ppm, cobalt (Co) ≤ 5 ppm, lead (Pb) ≤ 10 ppm, nickel (Ni) ≤ 5 ppm, manganese (Mn) ≤ 50 ppm, titanium (Ti) ≤ 10 ppm, magnesium (Mg) ≤ 50 ppm, aluminum (Al) ≤ 20 ppm, and sulfur (S) ≤ 50 ppm.

[0063] Table 2 Analysis and Detection Results of Iron Phosphate Products

[0064]

[0065]

[0066] Although the present invention has been described herein with reference to multiple illustrative embodiments of the present invention, it should be understood that those skilled in the art can design many other modifications and embodiments that will fall within the scope of the principles and spirit disclosed in this application. More specifically, within the scope of this application's disclosure, drawings, and claims, various deformations and improvements can be made to the components or layout of the subject combination layout. In addition to the deformations and improvements to the components or layout, other uses will also be obvious to those skilled in the art.

[0067] The foregoing has shown and described 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 by the above embodiments, and what is described in the above embodiments and the specification is only to illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will also have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing iron phosphate with low impurity content by using by - product ferrous sulfate of titanium dioxide, characterized in that, It includes the following steps: S1. Take ferrous sulfate by-produced from titanium dioxide and dissolve it in water to obtain a saturated solution of ferrous sulfate. While stirring, add an alkaline impurity remover to the saturated ferrous sulfate solution, control the pH of the solution at 4.0 - 6.0 for the impurity removal reaction. After the resulting slurry is filtered, a ferrous sulfate solution is obtained and then an acidic impurity remover is added for a secondary impurity removal reaction, control the pH of the solution at 2.0 - 3.

0. After the resulting slurry is filtered, a refined ferrous sulfate solution A is obtained, and it is diluted with water to a Fe 2+ concentration of 0.80 - 1.25 mol / L for standby; S2. Dissolve industrial-grade phosphate in water to prepare a phosphate solution with a concentration of 0.5 - 1.2 mol / L. Add a pH regulator to adjust the pH of the solution to 6.0 - 8.0 for impurity removal reaction. Add hydrogen peroxide with a certain excess coefficient to the filtered phosphate clear solution. After stirring evenly, obtain phosphate mixed solution B, and control the pH value of the mixed solution at 6.5 - 7.5; S3. Rapidly add the phosphate solution B obtained in step S2 to the ferrous sulfate solution A obtained in step S1, heat up to 45 - 60 °C for reaction, control the pH of the synthetic reaction slurry to be 2.5 ± 0.

5. After reacting for 20 - 30 min, a light yellow iron phosphate intermediate slurry C is obtained. Meanwhile, detect whether there is Fe in the slurry 2+ . If there is, add hydrogen peroxide to the slurry until there is no Fe 2+ ; S4. Filter the light yellow iron phosphate intermediate suspension C obtained in step S3, and repeatedly wash it with pure water at 60 - 70 °C until the conductivity of the washing water ≤ 5 ms / cm to obtain filter cake D; S5. Slurry the filter cake D obtained in step S3 with a certain amount of dilute phosphoric acid solution according to the solid content of 10.0 - 15.0% calculated as FePO4·2H2O to obtain suspension E. The concentration of the dilute phosphoric acid solution is 2.0 - 4.0% calculated as H3PO4. Control the ratio of total P to total Fe in the reaction system at 1.05 - 1.30:

1. After aging and crystal transformation reaction under high temperature and stirring conditions, obtain white iron phosphate slurry F; S6. Filter the iron phosphate slurry F obtained in step S5, and repeatedly wash it until the conductivity of the washing water ≤ 2500 μs / cm. The filter cake obtained after washing is dried to remove surface water, calcined at high temperature to remove free water, and then crushed to obtain anhydrous iron phosphate product. Take the washing water with conductivity ≤ 3500 during the washing process and mix it with concentrated phosphoric acid to prepare the dilute phosphoric acid solution required in step S5.

2. The method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In step S1, the basic impurity remover is one of ammonia water, sodium hydroxide, potassium hydroxide or calcium hydroxide, and the acidic impurity remover is one of phosphoric acid and sulfuric acid.

3. The method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 2, characterized in that: In step S1, the basic impurity remover is ammonia water, and the acidic impurity remover is phosphoric acid.

4. The method for synthesizing ferric phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In step S1, the temperature of the impurity removal reaction is 50 - 60 °C, and the reaction time is 30 min - 90 min.

5. The method for synthesizing ferric phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In step S2, the pH regulator is one of ammonia water, sodium hydroxide, and potassium hydroxide.

6. The method for synthesizing ferric phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 5, wherein: In step S2, the pH regulator is ammonia water.

7. The method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In step S2, the temperature of the impurity removal reaction is 40 - 50 °C, the reaction time is 20 min - 30 min, and the excess coefficient of hydrogen peroxide is 1.10 - 1.

40.

8. The method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In the step S3, the rapid feeding time of the phosphate mixed solution B is 5 to 15 minutes, and the indicator for detecting whether there is Fe 2+ in the detection slurry is bipyridine indicator.

9. The method for synthesizing iron phosphate with low impurity content by using by-product ferrous sulfate of titanium dioxide as claimed in claim 1, wherein: In step S3, the addition amount of the phosphate solution B is added according to the molar ratio of phosphate to iron salt of 0.85 - 0.95:1.

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