METHOD FOR THE PRODUCTION OF IRON(III) PHOSPHATE

Abstract

A process for the production of iron(III) phosphate, comprising the following steps: - Mixing a surfactant with a first metal solution containing iron and phosphorus elements; - Addition of seed crystals; - Aging by heating and stirring to obtain an aged solution; - Filtering the aged solution to obtain a filter residue; and - Drying and sintering of the filter residue to obtain the iron(III) phosphate; wherein the seed crystals are iron(III) phosphate dihydrate or basic ammonium iron(III) phosphate; wherein the surfactant is selected from a group consisting of cetyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium dodecylsulfonate and polyvinylpyrrolidone; and wherein the first metal solution is a filtrate obtained by dissolving waste iron(III) phosphate in acid and subsequent filtration, wherein the waste iron(III) phosphate is at least one of anhydrous iron(III) phosphate, iron(III) phosphate dihydrate, amorphous iron(III) phosphate and lithium-extracted waste lithium iron phosphate cathode powder.

Description

TECHNICAL FIELD

[0001] The present invention belongs to the field of battery materials and relates to a process for producing iron(III) phosphate. BACKGROUND

[0002] Lithium iron phosphate batteries are widely used by lithium battery manufacturers because they are inexpensive, low-toxic, highly safe, and long-lasting, and do not contain rare elements such as Ni and Co. The precursor for the production of lithium iron phosphate cathode materials is ferric phosphate, the quality of which directly affects the performance of lithium iron phosphate batteries. Current technology mainly uses iron salt as the iron source to form ferric phosphate, and an oxidizer such as hydrogen peroxide is added to oxidize the divalent iron ions to trivalent iron ions. During oxidation, a large amount of hydrogen peroxide is consumed as an oxidant, resulting in higher costs and lower benefits compared to materials such as ternary metals.Current technology typically produces ferric phosphate dihydrate using a two-step co-precipitation method using aqueous ammonia and NaOH as the alkaline solution and phosphoric acid as the aging agent. The slurry produced by this method has high viscosity and poor batch stability. The massive use of the alkaline solution increases production costs and tends to make water treatment more difficult.

[0003] CN 1 12 624 076 A discloses a process for producing ferric phosphate. According to the process, recycled ferric phosphate waste is used as a raw material, an amorphous ferric phosphate is precipitated by an alkaline solution, and an alkaline solution (ammonia water) and phosphoric acid are used as an aging agent under high-temperature stirring to obtain a basic ammonium ferric phosphate crystal powder, which is then roasted and dehydrated to obtain the ferric phosphate.

[0004] CN 1 12 456 460 A discloses a process for producing iron(III) phosphate by a surfactant-hydrothermal process. The process comprises the following steps: iron salt, phosphate, and surfactant are subjected to a hydrothermal reaction to obtain iron phosphate dihydrate, and the iron phosphate dihydrate is subsequently calcined and ball-milled to obtain nanometer-sized iron(III) orthophosphate.

[0005] CN 1 12 320 780 A discloses a recycling process for ferric phosphate waste. According to the process, ferric phosphate waste is leached to obtain an acidic leaching solution, which is then subjected to oxidation and precipitation to obtain a ferric phosphate filter residue. Subsequently, a battery-grade ferric phosphate is obtained through an aging process followed by calcination.

[0006] Battery-grade ferric phosphate contains fewer impurities and is of stable quality, and the lithium iron phosphate battery made from it has stable performance and high capacity. The impact of battery-grade ferric phosphate on the performance of lithium iron phosphate is even more pronounced. Lithium iron phosphate synthesized from ceramic or food-grade ferric phosphate has low capacity, and these ferrous phosphates are only suitable as raw materials for the production of high-quality ceramics or nutritional supplements. The main problems in the production of ferric phosphate are as follows: 1. The production of ferric phosphate from a divalent iron source requires an oxidizing agent, cannot guarantee the uniformity of oxidation and the oxidation time, and increases production costs. 2. The small-grain ferric phosphate contains SO4. 2-, which is difficult to wash out, easily clumps, and requires multiple washings to produce products with qualified purity. 3. Batch stability is poor, and the physical and chemical properties fluctuate greatly within batches because the intermittent method is widely used in the market, resulting in unguaranteed batch stability. Therefore, there is an urgent need to develop a low-cost, non-aggregation production process for ferric phosphate whose impurities can be easily washed out, which not only brings economic benefits to enterprises but also protects the environment. SUMMARY

[0007] The present invention aims to solve at least one of the above-mentioned technical problems of the prior art. Accordingly, the present invention provides a production process for ferric phosphate. The dehydrated ferric phosphate produced in the present invention is a spherical particle with a controllable and uniformly distributed particle size, high tap density, and controllable crystal type and morphology. It can be used as a precursor material for lithium iron phosphate with high compactness and also has good application prospects in ceramics and catalysts.

[0008] According to one aspect of the present invention there is provided a process for producing iron(III) phosphate, the process comprising: Mixing a surfactant with a first metal solution containing iron and phosphorus elements, adding seed crystals, aging with heating and stirring to obtain an aged solution, filtering the aged solution to obtain a filter residue, and drying and sintering the filter residue to obtain the ferric phosphate; wherein the seed crystals are ferric phosphate dihydrate or basic ammonium ferric phosphate; wherein the surfactant is at least one of cetyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium dodecylsulfonate, and polyvinylpyrrolidone; and wherein the first metal solution is a filtrate obtained by dissolving waste ferric phosphate in acid and then filtering, wherein the waste ferric phosphate is at least one of anhydrous ferric phosphate, ferric phosphate dihydrate, amorphous ferric phosphate, and lithium-extracted ferric phosphate cathode powder waste.A chemical formula of basic ammonium iron(III) phosphate is NH4Fe2(OH)(PO4)2·nH2O, where n is 1 or 2.

[0009] According to a preferred embodiment of the present invention, the steps for preparing the seed crystals are as follows: adding a first alkaline solution to a second metal solution containing iron and phosphorus elements, adjusting a pH value of a resulting solution, and subjecting the resulting solution to crystal conversion and aging while heating and stirring to obtain the seed crystals; wherein the molar ratio of iron to phosphorus in the second metal solution is 1:1.10 to 1:1.50.

[0010] According to a preferred embodiment of the present invention, waste ferric phosphate is dissolved in acid and filtered to obtain a filtrate. The second metal solution is the filtrate obtained by the acid dissolution and filtration; wherein the waste ferric phosphate is at least one material selected from the group consisting of anhydrous ferric phosphate, ferric phosphate dihydrate, amorphous ferric phosphate, or lithium-extracted waste lithium ferric phosphate cathode powder. By using waste ferric phosphate as raw material, waste resources are recycled cost-effectively, which not only brings economic benefits to enterprises but also protects the environment.

[0011] According to a preferred embodiment of the present invention, when the waste ferric phosphate is ferric phosphate dihydrate, the acidic dissolution is performed after a firing step, and the firing step is carried out at a temperature of 250°C to 450°C for 1 hour (h) to 5 hours. More preferably, the firing step is carried out at a temperature of 300°C to 400°C for 2 hours to 4 hours. The firing step serves to dehydrate ferric phosphate dihydrate into anhydrous ferric phosphate, thereby dissolving the ferric phosphate in an acidic solution.

[0012] According to a preferred embodiment of the present invention, the acid solution used in the acid dissolution is at least one selected from the group consisting of sulfuric acid, hydrochloric acid, and phosphoric acid with a concentration of 0.8 mol / l to 3 mol / l. More preferably, the acid solution is sulfuric acid with a concentration of 1.2 mol / l to 2.0 mol / l. Preferably, the mass concentration of the phosphoric acid is 80% to 90%, more preferably 85%.

[0013] In some preferred embodiments of the present invention, the acid dissolution is carried out at a temperature of 25°C to 90°C for 1 h to 10 h. More preferably, the acid dissolution is carried out at a temperature of 40°C to 70°C for 2 h to 5 h.

[0014] According to a preferred embodiment of the present invention, the molar ratio of iron to phosphorus in the first metal solution is 1:1.10 to 1:1.50, more preferably 1:1.15 to 1:1.30.

[0015] According to a preferred embodiment of the present invention, stirring is carried out at a speed of 150 rpm to 450 rpm, more preferably 200 rpm to 350 rpm, and heating is carried out at a temperature of 60 °C to 95 °C.

[0016] According to a preferred embodiment of the present invention, the aging is carried out for 1 h to 10 h, more preferably 2 h to 5 h.

[0017] According to a preferred embodiment of the present invention, the method further comprises a process of adding a second alkaline solution to adjust a pH of a resulting solution after addition with the seed crystals, wherein the pH of the solution is adjusted to 0.5 to 4, more preferably 2 to 3; wherein the second alkaline solution is a solution of at least one of the following substances: ammonium bicarbonate, ammonium carbonate, ammonium chloride, aqueous ammonia solution, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0018] According to a preferred embodiment of the present invention, the second alkaline solution is an aqueous solution of ammonium chloride or aqueous ammonia.

[0019] According to a preferred embodiment of the present invention, in the process for producing the seed crystals, the pH of a resulting solution is adjusted to 1.5 to 3.5, more preferably 1.5 to 2.5; wherein the first alkaline solution is a solution of at least one of the following substances: sodium hydroxide, potassium hydroxide, sodium bicarbonate, ammonium bicarbonate, sodium carbonate, aqueous ammonia solution, and potassium carbonate.

[0020] According to a preferred embodiment of the present invention, the mass concentration of the first alkaline solution is 10% to 30%. More preferably, the first alkaline solution is an aqueous solution of sodium hydroxide or aqueous ammonia with a concentration of 20% to 25%.

[0021] According to a preferred embodiment of the present invention, the mass of the surfactant is 0.1% to 2% of the mass of the iron in the first metal solution.

[0022] According to a preferred embodiment of the present invention, the surfactant is one selected from the group consisting of cetyltrimethylammonium bromide, sodium dodecylsulfonate, and polyvinylpyrrolidone. More preferably, the mass of the surfactant is 0.5% to 1% of the mass of the iron in the first metal solution.

[0023] According to a preferred embodiment of the present invention, drying is carried out at a temperature of 90°C to 190°C for 6 h to 24 h. More preferably, drying is carried out at a temperature of 100°C to 140°C for 12 h to 15 h.

[0024] According to a preferred embodiment of the present invention, the method further comprises a step of washing the filter residue with water prior to drying, and the washing with water is carried out until the electrical conductivity of the water used is below 500 µS / cm. More preferably, the washing with water is carried out until the electrical conductivity of the water used is below 200 µS / cm.

[0025] According to a preferred embodiment of the present invention, sintering is carried out in an atmosphere consisting of at least one substance selected from the group consisting of air, nitrogen gas, and argon gas. The temperature increase rate of sintering is 2°C / min to 15°C / min, and sintering is carried out at 200°C to 350°C for 1 to 3 hours, followed by 500°C to 650°C for 2 to 6 hours.

[0026] According to a preferred embodiment of the present invention, the process for producing iron(III) phosphate comprises the following steps: (1) Calcining waste ferric phosphate and then adding an acidic solution for acidic dissolution, followed by filtration to obtain a filtrate which is the metal solution containing iron and phosphorus, and measuring the amounts of iron and phosphorus in the metal solution; (2) Taking the metal solution into a first reaction vessel as a bottom solution, supplying the metal solution and a first alkaline solution together to the first reaction vessel while stirring the solution in the first reaction vessel, adjusting a pH value of a resulting mixed solution and supplementing phosphorus or iron according to the amounts of iron and phosphorus in the metal solution and a total volume of the metal solution, heating a resulting solution for crystal conversion and aging to obtain a slurry containing seed crystals; (3) adding a surfactant to a second reaction vessel containing the metal solution, pumping the slurry containing the seed crystals into the second reaction vessel, feeding the slurry containing the seed crystals and a second alkaline solution together to the second reaction vessel while stirring the solution in the second reaction vessel; (4) controlling the pH of a resulting solution in the second reaction vessel after the slurry containing the seed crystals is completely added, heating for aging, washing and filtering a resulting solution to obtain a filter residue, and drying the filter residue; (5) Sintering a powdered filter residue obtained by drying in step (4) to obtain the iron(III) phosphate.

[0027] According to a preferred embodiment of the present invention, the volume of the first reaction vessel is 50 l to 500 l, more preferably 300 l to 500 l.

[0028] According to a preferred embodiment of the present invention, the stirrer used for stirring in each of steps (2) and (3) is a four-blade screw stirrer, a four-blade straight-blade open-turbine type stirrer, a six-blade open-turbine type inclined stirrer, a six-blade straight-blade disc-turbine type stirrer, or a six-blade disc-turbine type inclined stirrer. More preferably, the stirrer is a four-blade straight-blade open-turbine type stirrer or a six-blade straight-blade disc-turbine type stirrer.

[0029] According to a preferred embodiment of the present invention, in step (2), the volume of the soil solution is 1 / 5 to 1 / 3, preferably 1 / 5, of the volume of the first reaction vessel.

[0030] According to a preferred embodiment of the present invention, in step (2), the ratio of the feed rate of the metal solution to the feed rate of the first alkaline solution is 10:1 to 3:1, more preferably 10:1 to 8:1.

[0031] According to a preferred embodiment of the present invention, in step (2), the heating temperature is 70 °C to 95 °C and the aging time is 3 h to 10 h.

[0032] Preferably, the heating temperature is 80 °C to 95 °C and the aging time is 3 h to 5 h.

[0033] According to a preferred embodiment of the present invention, in step (3), the volume of the second reaction vessel is 500 l to 10,000 l, more preferably 1,000 l to 5,000 l.

[0034] According to a preferred embodiment of the present invention, in step (3), the metal solution in the second reaction vessel is 50% to 90%, more preferably 60% to 80%, of the volume of the second reaction vessel.

[0035] According to a preferred embodiment of the present invention, in step (3), the ratio of the feed rate of the second alkaline solution to the feed rate of the slurry containing the seed crystals is 1 : 1.5 to 0 : 3.5, preferably 0.5 : 1.5 to 0 : 2.5.

[0036] According to a preferred embodiment of the present invention, the powdered filter residue obtained by drying in step (4) is iron(III) phosphate dihydrate or basic ammonium iron(III) phosphate.

[0037] According to a preferred embodiment of the present invention, in step (4), the heating temperature is 60°C to 95°C and the aging time is 3 h to 10 h. Furthermore, the heating temperature is 80°C to 95°C and the aging time is 2 h to 5 h.

[0038] According to a preferred embodiment of the present invention, the present invention has at least the following advantageous effects. 1. In the present invention, by adding a small amount of presynthesized ferric phosphate dihydrate or basic ammonium ferric phosphate as seed crystals to the entire ferric phosphate reaction system, the thermodynamic barrier to nucleation in the reaction system can be reduced. Thus, a pure phase of ferric phosphate dihydrate or basic ammonium ferric phosphate can be formed without adding an alkaline solution, which accelerates the formation of a well-crystallized product, shortens the synthesis time of ferric phosphate dihydrate or basic ammonium ferric phosphate, and avoids the formation of multiple morphologies of ferric phosphate when seed crystals are not added.The driving force for the crystallization of the iron(III) phosphate in the present invention is the seed crystals, not the phosphoric acid that causes the amorphous product to crystallize in the conventional aging process. The resulting product exhibits a very uniform morphology and particle size. 2. In the present invention, the surfactant is used to modify the seed crystals to improve the surface activity of the seed crystals. By inducing epitaxial Fe 3+ - and PO4 3-Growth on the surface of the seed crystals generates secondary crystal nuclei, which lead to the formation of the basic framework of the product particles. Through the aging process, the deposition of crystal nuclei on the surface of the seed crystals makes the framework of the crystal grain more complete, so that the primary particles are arranged more densely and orderly, and tend to form spherical secondary particles. The ultimately formed anhydrous ferric phosphate has a particle size D50 of approximately 2 µm to 30 µm. The particles are controllable and easy to wash, have less moisture, and are easy to dry. The secondary particles have a uniform morphology and a higher tap density, making them suitable for the production of lithium iron phosphate batteries with high compactness. 3. The equipment required for the present invention is simple and easy to operate. The production process eliminates repeated washing operations, resulting in less wastewater and lower water treatment costs. With such a ferric phosphate produced by a semi-continuous process, the poor consistency between different product batches is resolved and batch stability of the products is ensured. 4. In the present disclosure, ferric phosphate dihydrate and basic ammonium ferric phosphate can be selectively produced, which is then sintered to obtain anhydrous ferric phosphate. This process requires less consumption of phosphoric acid and alkaline solution and is therefore more cost-effective compared to the pure intermittent alkaline precipitation method. BRIEF DESCRIPTION OF THE CHARACTERS

[0039] The present invention is described in more detail below in conjunction with the attached figures and embodiments. Fig. 1 is a flowchart of a method according to Embodiment 2 of the present invention. Fig. 2 is a schematic diagram of a microcosmic reaction process according to Embodiment 1 of the present invention. Fig. 3 is an X-ray diffraction pattern (XRD) of iron(III) phosphate dihydrate formed according to Embodiment 1 of the present invention. Fig. 4 is a scanning electron microscopy (SEM) image of ferric phosphate dihydrate formed according to Embodiment 1 of the present invention. Fig. 5 is an XRD pattern of anhydrous ferric phosphate formed according to Embodiment 1 of the present invention. Fig.is an SEM image of anhydrous ferric phosphate formed according to Embodiment 1 of the present invention. Fig. 7 is a graph showing the charge-discharge curves at 0.1 C of lithium iron phosphate formed from anhydrous iron phosphate according to Embodiment 1 of the present invention. DETAILED DESCRIPTION

[0040] The technical solutions and their effects are described clearly and completely using the exemplary embodiments of the present invention in order to clarify the object, technical features, and advantages of the present invention. Obviously, the described exemplary embodiments represent only a part and not all embodiments of the present invention. Starting from the exemplary embodiments of the present description, other exemplary embodiments also fall within the scope of the present invention, which a person skilled in the art can obtain without inventive step based on the exemplary embodiments of the present description. Example 1

[0041] In this example, iron(III) phosphate was prepared using the following procedure: (1) 100 kg of waste ferric phosphate dihydrate was fired at 300 °C for 4 hours to remove crystalline water, leaving about 80 kg of fired material. 80 kg of the fired material was placed in a kettle containing 666 l of a 1.2 mol / l sulfuric acid solution and stirred at 300 rpm, heated to 60 °C for about 6 h to dissolve it, and allowed to stand. After using a precision filter to filter out the filter residues, the resulting filtrate, which contained a metal solution A containing Fe 3+ and PO4 3- was transferred to a storage tank. The concentrations of the elements Fe and P in metal solution A were 40.51 g / l and 22.96 g / l, respectively. The molar ratio of Fe to P in metal solution A was 1:1.022. (2) 20 L of metal solution A was injected as the bottom solution into a 100-liter reaction vessel P1 and stirred at 300 rpm. The metal solution A and a NaOH solution were co-fed into the reaction vessel P1 at a feed rate of 50 L / h and 7.5 L / h, respectively. A real-time pH feedback system fine-tuned the feed rate of the NaOH solution. The temperature of the reaction vessel P1 was set to 40 °C. At the end of the co-feed step, the pH of the solution in the reaction vessel P1 was adjusted to 1.8 to precipitate amorphous iron phosphate. The total metal solution A in the reaction vessel P1 was 80 L (including the bottom solution A and the co-fed metal solution A). 0.435 l of phosphoric acid was added to the reaction vessel P1 until the molar ratio of Fe to P was 1:1.20, thereby obtaining slurry B.Slurry B was heated to 93 °C, maintained at this temperature for 2.5 h to achieve crystal transformation, and aged for 5 h to obtain Slurry C. The residual Fe content in the filtrate when Slurry C was filtered was 35 mg / L. (3) 600 l of metal solution A were placed in a reaction vessel P2 with a volume of 1 m 3 injected. While stirring at 250 rpm, 24 g of polyvinylpyrrolidone was added as a surfactant, and slurry C was pumped from reaction vessel P1 to reaction vessel P2 at a rate of 100 l / h. (4) After the ferric phosphate dihydrate slurry C was completely added, the pH of the solution in the reaction vessel P2 was 0.8. The reaction vessel P2 was heated to 85 °C and then aged for 3 hours. The resultant was washed with water and filtered until the electrical conductivity of the used water was below 400 µS / cm. The filter residue was collected to obtain a ferric phosphate dihydrate filter cake D, which was dried at 100 °C for 15 h to obtain ferric phosphate dihydrate powder E. (5) The dried ferric phosphate dihydrate powder E was placed in a muffle furnace, the temperature of which was increased to 350 °C at 10 °C / min and maintained at this temperature for 3 hours. Then, the temperature was increased to 550 °C at 10 °C / min and sintered for 3 hours. The resulting product was then naturally cooled to room temperature to obtain the qualified anhydrous FePO4. Finally, the obtained product was subjected to phase and performance testing and analysis. The impurity content of the anhydrous ferric phosphate was 0.0057 wt%.

[0042] The physical and chemical properties of the iron(III) phosphate dihydrate and anhydrous iron(III) phosphate obtained in this embodiment are listed in Table 1. Table 1. Physical and chemical properties of iron(III) phosphate dihydrate and anhydrous iron(III) phosphate Iron(III) phosphate dihydrate Fe wt%) P (wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 29.01 16.46 0.977 4.43 6.88 10.7 0.911 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 21.4 1.08 0.0009 0.0002 0.0005 0.0002 0.0001 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0018 0.0001 0.0001 0.0055 0.0001 0.0009 0.0001 anhydrous iron(III) phosphate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 36.36 20.61 0.978 4.35 6.95 11.54 1.035 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 5.41 1.19 0.0012 0.0001 0.0001 0.0001 0.0003 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0015 0.0001 0.0001 0.0005 0.0001 0.0015 0.0001

[0043] Table 1 shows that the iron and phosphorus contents, as well as the contents of other elements, in ferric phosphate dihydrate and anhydrous ferric phosphate comply with the Chinese standards for ferric phosphate. The particle size distribution is narrow, the particle size distribution is small, the tap densities before and after sintering are both high, and the specific surface area values ​​are moderate. Therefore, the ferric phosphate product is suitable as a precursor material for the production of lithium iron phosphate with high compactness.

[0044] By adding a small amount of ferric phosphate dihydrate, pre-synthesized in reaction vessel P1 as seed crystals, to reaction vessel P2 of the second reaction system, the ferric phosphate dihydrate can reduce the thermodynamic barrier for crystal nucleation in the reaction system. In reaction vessel P2, a pure phase of ferric phosphate dihydrate is obtained without the addition of an alkaline solution, which accelerates the formation of a well-crystallized product, shortens the time for the synthesis of ferric phosphate dihydrate, and avoids the formation of ferric phosphate in multiple morphological forms when no seed crystals are added. The function of the seed crystals is to provide crystallization nuclei and initiate crystallization. The crystallization process is carried out according to the procedure described in Fig.2. The driving force for the crystallization of the iron(III) phosphate is the seed crystals and not the phosphoric acid, which causes the amorphous product to crystallize in the conventional aging process. The formed product has a very uniform morphology and particle size. The surfactant in the reaction vessel is used to modify the seed crystals to improve their surface activity. By epitaxially growing Fe 3+ - and PO4 3--Growth on the surface of the seed crystal generates secondary crystal nuclei, which cause the formation of the basic framework of the product particles. Through the aging process, the deposition of crystal nuclei on the surface of the seed crystal makes the framework of the crystal grain more complete, so that the primary particles are arranged more densely and orderly, and tend to form spherical secondary particles. During the epitaxial growth process, the primary particles always grow in the direction of the shear force due to the shearing action of the tangential flows generated by the stirrer, thus forming a sheet-like structure. Compared with the method that uses an alkaline solution for precipitation and phosphoric acid for aging, the present method does not require a high pH value, so the production can be carried out under strongly acidic conditions.No phosphoric acid solution is needed to provide the driving force for crystallization, thereby shortening the time for transformation from amorphous to crystalline state, and thus only a short period of aging is required to make the crystal form more complete and the precipitation more thorough, so that the alkaline solution is less consumed and the yield is high.

[0045] Fig. and Fig. show the XRD pattern and the SEM image of the iron(III) phosphate dihydrate prepared in Example 1. Fig. 3 shows that the iron(III) phosphate dihydrate prepared in Example 1 has a relatively high purity and good crystallinity, and no impurity phases were found. Fig.4 shows that the produced iron(III) phosphate dihydrate particles have a uniform particle size distribution, good consistency of the secondary particles and good particle dispersion.

[0046] Fig. and Fig. show the XRD pattern and the SEM image of the anhydrous iron(III) phosphate prepared in Example 1. Fig. 5 shows that the anhydrous iron(III) phosphate prepared in Example 1 has a relatively high purity and good crystallinity, and no impurity phases were found. Fig. 6 shows that the prepared anhydrous iron(III) phosphate particles have a uniform particle size distribution, good consistency of the secondary particles and good particle dispersion.

[0047] Fig.Figure 7 is a graph showing the charge and discharge curves at 0.1 C of lithium iron phosphate synthesized from the anhydrous ferric phosphate precursor in Working Example 1. It can be seen that the lithium iron phosphate prepared using the anhydrous ferric phosphate from Working Example 1 as a precursor has initial charge and discharge capacities of 161.4 mAh / g and 158.4 mAh / g, respectively, showing similar electrochemical performance to commercially available products. Example 2

[0048] In this embodiment, which is based on Fig. 1, iron(III) phosphate was produced by the following process: (1) 100 kg of waste ferric phosphate dihydrate was calcined at 400 °C for 3 hours to remove crystalline water, leaving about 80 kg of calcined material. 72 kg of the calcined material was placed in a kettle containing 600 L of a 1.2 mol / L sulfuric acid solution and stirred at 200 rpm. The calcined material was heated to 45 °C for about 8 hours to dissolve it, and then allowed to stand. After using a precision filter to filter out the filter residue, the resulting filtrate, a metal solution A, was transferred to a storage tank. The concentrations of the elements Fe and P in the metal solution A were 42.51 g / L and 24.35 g / L, respectively. The molar ratio of Fe to P in the metal solution A was 1:1.033. (2) 10 L of metal solution A was injected as a bottom solution into a 50-liter reaction vessel P1 and stirred at 350 rpm. The metal solution A and a NaOH solution were co-fed into the reaction vessel P1 at a feed rate of 40 L / h and 6 L / h, respectively. A real-time pH feedback system fine-tuned the feed rate of the NaOH solution. The temperature of the reaction vessel P1 was set to 45 °C. At the end of the co-feed step, the pH of the solution in the reaction vessel P1 was adjusted to 2.1 to precipitate amorphous iron phosphate. The total metal solution A in the reaction vessel P1 was 45 L (including the bottom solution A and the co-flowing metal solution A). 0.687 l of phosphoric acid was added to the reaction vessel P1 until the molar ratio of Fe to P was 1:1.30, yielding a slurry B of iron(III) phosphate dihydrate.Slurry B was heated to 90°C, held at this temperature for 3 hours to allow crystal transformation, and aged for 3 hours to obtain ferric phosphate dihydrate slurry C. The residual Fe content in the filtrate after filtering off slurry C was 1.05 mg / L. (3) 400 l of metal solution A were placed in a reaction vessel P2 with a volume of 0.5 m 3 injected. While stirring at 300 rpm, 85 g of sodium dodecylsulfonate was added as a surfactant, and ferric phosphate dihydrate slurry C was pumped from reaction vessel P1 to reaction vessel P2 at a rate of 100 l / h. The feed rate of the second alkaline solution was 10 l / h. (4) After complete addition of the ferric phosphate dihydrate slurry C, the pH of the solution in reaction vessel P2 was 3. The reaction vessel P2 was heated to 85 °C and then aged for 3 h. The resultant was washed with water and filtered until the electrical conductivity of the used water was below 500 µS / cm. The filter residue was collected to obtain a filter cake D, which was dried at 120 °C for 12 hours to obtain basic ammonium ferric phosphate powder E. (5) The dried basic ammonium iron(III) phosphate powder E was placed in a muffle furnace, the temperature of which was increased to 400 °C at 10 °C / min and maintained at this temperature for 2 hours. Then, the temperature was increased to 600 °C at 5 °C / min and sintered for 2 hours. The resulting product was then naturally cooled to room temperature to obtain the qualified anhydrous FePO4. Finally, the obtained product was subjected to phase and performance testing and analysis. The impurity content of the iron phosphate was 0.0152 wt%.

[0049] The physical and chemical properties of the basic ammonium iron(III) phosphate and anhydrous iron(III) phosphate obtained in this embodiment are listed in Table 2. Table 2. Physical and chemical properties of basic ammonium iron(III) phosphate and anhydrous iron(III) phosphate Basic ammonium iron phosphate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 29.21 16.66 0.972 5.35 7.56 11.3 0.787 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 19.57 1.35 0.0011 0.0001 0.0012 0.0001 0.0001 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0036 0.0011 0.0001 0.0359 0.0001 0.0015 0.0001 Anhydrous iron(III) phosphate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 36.36 20.63 0.977 15.01 19.66 28.11 0.661 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 2.31 1.40 0.0006 0.0001 0.0009 0.0001 0.0002 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0045 0.0009 0.0001 0.0055 0.0001 0.0021 0.0001

[0050] In embodiment 2, the supersaturated iron(III) phosphate dihydrate can reduce the thermodynamic barrier for crystal nucleation by adding a small amount of iron(III) phosphate dihydrate, which was presynthesized in the reaction vessel P1 as seed crystals, to the reaction vessel P2, NH4 + , Fe 3+ and PO4 3-to grow epitaxially on the surface of the seed crystals, thereby growing a new basic ferric ammonium phosphate secondary crystal nucleus on the surface of the seed crystal and accelerating the formation of new crystal nuclei with good crystallinity. There is no need to add excess phosphoric acid for aging, thereby reducing the amount of phosphoric acid used, shortening the aging time, and reducing energy consumption. It can be seen from Table 2 that the basic ammonium ferric phosphate prepared in Working Example 2 has relatively high purity and relatively good particle dispersion. After sintering, the anhydrous ferric phosphate has good crystallinity. The contents of iron and phosphorus and the contents of other elements in the basic ammonium ferric phosphate and anhydrous ferric phosphate comply with the Chinese standards for ferric phosphate.The anhydrous iron(III) phosphate has a tap density of 1.40 g / cm. 3 and a specific surface area of ​​2.31 m 2 / g. Therefore, iron(III) phosphate is suitable as a starting material for the production of lithium iron phosphate with high compactness. Example 3

[0051] In this example, iron(III) phosphate was prepared using the following procedure: (1) 100 kg of waste ferric phosphate dihydrate was calcined at 300 °C for 3 hours to remove crystal water, leaving about 80 kg of calcined material. 72 kg of the calcined material was placed in a kettle containing 600 L of a 1.2 mol / L sulfuric acid solution and stirred at 250 rpm, heated to 60 °C for about 2 hours to dissolve, and allowed to stand. After using a precision filter to filter out the filter residue, the obtained filtrate, a metal solution A, was transferred to a storage tank. The concentrations of the elements Fe and P in the metal solution A were 42.01 g / L and 24.10 g / L, respectively. The molar ratio of Fe to P in the metal solution A was 1:1.035. (2) 20 L of metal solution A was injected as the bottom solution into a 100-L reaction vessel P1 and stirred at 250 rpm. The metal solution A and aqueous ammonia were introduced into the reaction vessel P1 at a feed rate of 40 L / h and 6 L / h, respectively. The feed rate of the aqueous ammonia was fine-tuned using a real-time pH feedback system. The temperature of the reaction vessel P1 was set to 40 °C. At the end of the co-flowing step, the pH of the solution in the reaction vessel P1 was adjusted to 2.1 to precipitate amorphous iron phosphate. The total metal solution A in the reaction vessel P1 was 45 L (including the bottom solution A and the co-flowing metal solution A). 0.68 L of phosphoric acid was added to the reaction vessel P1 until the molar ratio of Fe to P was 1:1.30, obtaining slurry B.Slurry B was heated to 90°C, held at this temperature for 3 h for crystal transformation, and aged for 3 h, yielding basic ammonium iron(III) phosphate slurry C. The residual Fe content in the filtrate after filtration of slurry C was 1.05 mg / L. (3) 400 l of metal solution A were placed in a reaction vessel P2 with a volume of 0.5 m 3 injected. While stirring at 300 rpm, 85 g of sodium dodecylsulfonate was added as a surfactant, and the slurry C of basic ammonium iron(III) phosphate was pumped from reaction vessel P1 to reaction vessel P2 at a rate of 100 l / h. (4) After the basic ammonium ferric phosphate slurry C was completely added, the pH of the solution in the reaction vessel P2 was 0.95. The reaction vessel P2 was heated to 85 °C and then aged for 3 hours. The resultant was washed with water and filtered until the electrical conductivity of the used water was below 500 µS / cm. The filter residue was collected to obtain a ferric phosphate dihydrate filter cake D, which was dried at 120 °C for 12 hours to obtain ferric phosphate dihydrate powder E. (5) The dried ferric phosphate dihydrate powder E was placed in a muffle furnace, the temperature of which was increased to 400 °C at 10 °C / min and maintained at this temperature for 1 hour. Then, the temperature was increased to 550 °C at 5 °C / min and sintered at this temperature for 2 hours. The resulting product was then naturally cooled to room temperature to obtain the qualified anhydrous FePO4. Finally, the obtained product was subjected to phase and performance testing and analysis. The impurity content of the anhydrous iron phosphate was 0.0042 wt%.

[0052] The physical and chemical properties of the iron(III) phosphate dihydrate and anhydrous iron(III) phosphate obtained in this embodiment are listed in Table 3. Table 3. Physical and chemical properties of iron(III) phosphate dihydrate and anhydrous iron(III) phosphate dihydrate Iron(III) phosphate dihydrate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 29.12 16.54 0.976 6.58 9.86 15.44 0.899 BET(m 2 / g) TD(g / cc) Ni (wt.%) Co(wt%) Mn (wt.%) Ca(wt%) Mg (wt.%) 25.31 1.13 0.0001 0.0004 0.0002 0.0001 0.0002 Na(wt%) Cu(wt%) Zn (wt.%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0006 0.0001 0.0001 0.0125 0.0008 0.0001 0.0005 Anhydrous iron(III) phosphate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D5 0 36.25 20.63 0.974 10.41 16.01 25.23 0.926 BET(m 2 / g) TD(g / cc) Ni (wt.%) Co(wt%) Mn (wt.%) Ca(wt%) Mg (wt.%) 4.05 1.21 0.0004 0.0001 0.0001 0.0002 0.0002 Na(wt%) Cu(wt%) Zn (wt.%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0021 0.0001 0.0002 0.0004 0.0001 0.0001 0.0002

[0053] In embodiment 3, by adding a small amount of basic ammonium iron(III) phosphate, which was pre-synthesized in the reaction vessel P1 as seed crystals, to the reaction vessel P2 with the second reaction system, supersaturated basic ammonium iron(III) phosphate can reduce the thermodynamic barrier of crystal nucleation, which leads to Fe 3+ and PO4 3- grow epitaxially on the surface of the seed crystals. However, since no ammonium-containing alkaline solution is added as a starting material to increase the pH in this process, the pH in the system is too low, so NH4 +in the basic ammonium iron(III) phosphate complex, and the iron(III) phosphate skeleton structure of the basic iron(III) phosphate remains stable. In addition, sodium dodecylsulfonate is used as a surfactant, which improves the surface activity of the iron(III) phosphate skeleton structure, thereby generating new secondary iron(III) phosphate dihydrate crystal nuclei on the surface of the seed crystals and accelerating the formation of new crystal nuclei with good crystallinity. Excessive phosphoric acid does not need to be added for aging, thereby reducing the amount of phosphoric acid used, shortening the aging time, and reducing energy consumption. In the synthesis of iron(III) phosphate dihydrate using basic ammonium iron(III) phosphate as seed crystals, NH4 is used. + and OH -dissolved and leached out from the structural unit of basic ammonium iron(III) phosphate (NH4Fe2(OH)(PO4)2·nH2O), but the basic skeletal structure FePO4·2H2O is retained at the beginning of the addition of basic ammonium iron(III) phosphate in high acidity. Therefore, the remaining skeleton of basic ammonium iron(III) phosphate has a porous structure, which induces new nuclei to grow epitaxially on the surface of the seed crystals to form a porous structure, and thus the formed iron(III) phosphate dihydrate has a porous structure, which promotes the migration of lithium ions in the resulting lithium iron(III) phosphate, so that the produced lithium iron(III) phosphate has a high tap density and a high specific capacity. It can be seen from Table 3 that the iron phosphate dihydrate produced in Working Example 3 has a relatively high purity and relatively good particle dispersion.After sintering, the anhydrous ferric phosphate exhibits good crystallinity. The iron and phosphorus contents, as well as the contents of other elements in ferric phosphate dihydrate and anhydrous ferric phosphate, comply with Chinese standards for ferric phosphate. The anhydrous ferric phosphate has a tap density of 1.21 g / cm³. 3 and a specific surface area of ​​4.05 m 2 / g. Therefore, the iron(III) phosphate product is suitable as a starting material for the production of lithium iron phosphate with high compactness. Example 4

[0054] In this example, iron(III) phosphate was prepared using the following procedure: (1) 200 kg of lithium-extracted waste lithium iron phosphate cathode powder was fired at 350 °C for 3 h to remove crystalline water, leaving about 200 kg of fired material. 200 kg of the fired material was placed in a kettle containing 1000 L of a 1.5 mol / L sulfuric acid solution and stirred at 400 rpm, heated to 60 °C for about 3 h to dissolve, and allowed to stand. After using a precision filter to filter out the filter residue, the obtained filtrate, which was Metal Solution A, was transferred to a storage tank. The concentrations of the elements Fe and P in Metal Solution A were 48.12 g / L and 27.52 g / L, respectively. The molar ratio of Fe to P in Metal Solution A was 1:1.031. (2) 25 L of metal solution A was injected as a bottom solution into a 100-liter reaction vessel P1 and stirred at 300 rpm. The metal solution A and a NaOH solution were co-fed into the reaction vessel P1 at a feed rate of 100 L / h and 14 L / h, respectively. A real-time pH feedback system fine-tuned the feed rate of the NaOH solution. The temperature of the reaction vessel P1 was adjusted to room temperature. At the end of the co-feed step, the pH of the solution in the reaction vessel P1 was adjusted to 2.5 to precipitate amorphous iron phosphate. The total metal solution A in the reaction vessel P1 was 80 L (including the bottom solution A and the co-flowing metal solution A). 0.548 l of phosphoric acid was added to the reaction vessel P1 until the molar ratio of Fe to P was 1:1.15, forming a slurry B of iron(III) phosphate.Slurry B was heated to 95°C, maintained at this temperature for 3.5 hours for crystal transformation, and aged for 6 hours to obtain Slurry C. The residual Fe content in the filtrate when Slurry C was filtered off was 0.95 mg / L. (3) 800 l of metal solution A were placed in a reaction vessel P2 with a volume of 1 m 3 injected. While stirring at 250 rpm, 345 g of cetyltrimethylammonium bromide was added as a surfactant, and slurry C was pumped from reaction vessel P1 to reaction vessel P2 at a rate of 100 l / h. (4) After the slurry C was completely added, the pH of the solution in the reaction vessel P2 was 0.85. The reaction vessel P2 was heated to 80 °C and then aged for 4 hours. The resultant was washed with water and filtered until the electrical conductivity of the used water was below 200 µS / cm. The filter residue was collected to obtain a ferric phosphate dihydrate filter cake D, which was dried at 140 °C for 10 hours to obtain ferric phosphate dihydrate powder E. (5) The dried ferric phosphate dihydrate powder E was placed in a muffle furnace, the temperature of which was increased to 400 °C at 5 °C / min and maintained at this temperature for 2 hours. Then, the temperature was increased to 550 °C at 10 °C / min and sintered at this temperature for 3 hours. The resulting product was then naturally cooled to room temperature to obtain the qualified anhydrous FePO4. Finally, the obtained product was subjected to phase and performance testing and analysis. The impurity content of the anhydrous iron phosphate was 0.0163 wt%.

[0055] The physical and chemical properties of the iron(III) phosphate dihydrate and anhydrous iron(III) phosphate obtained in this embodiment are listed in Table 4. Table 4. Physical and chemical properties of iron(III) phosphate dihydrate and anhydrous iron(III) phosphate Iron(III) phosphate dihydrate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 29.04 16.39 0.983 3.10 4.57 6.67 0.781 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 21.5 1.11 0.0021 0.0017 0.0021 0.0001 0.0012 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0047 0.0001 0.0005 0.0132 0.0001 0.0021 0.0001 Anhydrous iron(III) phosphate Fe(wt%) P(wt%) Fe / P D10(um) D50(um) D90(um) (D90-D10) / D50 36.45 20.71 0.976 3.03 4.86 7.01 0.819 BET(m 2 / g) TD(g / cc) Ni(wt%) Co(wt%) Mn (wt%) Ca(wt%) Mg(wt%) 4.63 1.23 0.0013 0.0017 0.0025 0.0002 0.0008 Na(wt%) Cu(wt%) Zn (wt%) S(wt%) Al (wt%) Ti(wt%) Mo(wt%) 0.0034 0.0015 0.0013 0.0005 0.0001 0.0029 0.0001

[0056] Table 4 shows that the iron(III) phosphate dihydrate and the anhydrous iron(III) phosphate prepared in Example 4 exhibit good crystallinity. The iron and phosphorus contents, as well as the contents of other elements, comply with the Chinese standards for iron(III) phosphate. The anhydrous iron(III) phosphate has a tap density of 1.23 g / cm³. 3 and a specific surface area of ​​4.63 m 2 / g. Therefore, iron(III) phosphate is suitable as a starting material for the production of lithium iron phosphate with high compactness. Examples of tests

[0057] The ferric phosphate prepared in Examples 1-4 and the commercially available ferric phosphate were processed into lithium iron phosphate under the same conditions using a conventional method, and the compact density and other electrical properties of the resulting lithium iron phosphate were tested. The results are shown in Table 5 below. Table 5 Compacted density (g / cm) 3 Specific capacity of charge at 0.1C (mAh / g) Specific discharge capacity at 0.1C (mAh / g) Original efficiency (%) Example 1 2.421 161.4 158.4 98.12 Example 2 2.375 160.8 158.2 98.38 Example 3 2.388 161.3 158.5 98.26 Example 4 2.393 161.6 158.0 97.77 Commercially available product 2.382 160.1 157.2 98.19

[0058] From Table 5, it is clear that the compacted density and electrical properties of the lithium iron phosphate powders prepared from the anhydrous iron phosphate synthesized in Working Examples 1-4 of the present invention are similar to those of the lithium iron phosphate powder prepared from the commercially available iron phosphate, indicating that the iron phosphate synthesized in the present invention reaches the standard of anhydrous iron phosphate for battery-grade lithium iron phosphate.

Claims

[1] A process for producing iron(III) phosphate, comprising the following steps: - Mixing a surfactant with a first metal solution containing iron and phosphorus elements; - Addition of seed crystals; - Aging with heating and stirring to obtain an aged solution; - filtering the aged solution to obtain a filter residue; and - drying and sintering the filter residue to obtain the ferric phosphate; the seed crystals being ferric phosphate dihydrate or basic ammonium ferric phosphate; wherein the surfactant is selected from a group consisting of cetyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium dodecylsulfonate and polyvinylpyrrolidone; and wherein the first metal solution is a filtrate obtained by dissolving waste ferric phosphate in acid and then filtering, wherein the waste ferric phosphate is at least one of anhydrous ferric phosphate, ferric phosphate dihydrate, amorphous ferric phosphate, and lithium-extracted waste lithium iron phosphate cathode powder. [2] The method of claim 1, further comprising the following steps for preparing the seed crystals: - Addition of a first alkaline solution to a second metal solution containing iron and phosphorus elements; - Adjusting the pH of the resulting solution; and - carrying out a crystal transformation and aging the resulting solution with heating and stirring to obtain the seed crystals; wherein the molar ratio of iron to phosphorus in the second metal solution is 1:1.10 to 1:1.

50. [3] The method according to claim 2, wherein the second metal solution is a filtrate obtained by dissolving waste ferric phosphate in acid and then filtering; wherein the waste ferric phosphate is selected from a group consisting of anhydrous ferric phosphate, ferric phosphate dihydrate, amorphous ferric phosphate, and lithium-extracted waste lithium iron phosphate cathode powder. [4] The process of claim 1, wherein the molar ratio of iron to phosphorus in the first metal solution is 1:1.10 to 1:1.

50. [5] A process according to claim 1 or 2, wherein the stirring is carried out at a speed of 150 rpm to 450 rpm and the heating is carried out at a temperature of 60 °C to 95 °C. [6] The method according to claim 1, further comprising a step of adding a second alkaline solution to adjust a pH of a solution resulting after the addition with the seed crystals, wherein the pH of the resulting solution is adjusted to 0.5 to 4; wherein the second alkaline solution is a solution of at least one member selected from the group consisting of ammonium bicarbonate, ammonium carbonate, ammonium chloride, aqueous ammonia, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. [7] The method according to claim 2, wherein the pH of the resulting solution is adjusted to 1.5 to 3.5; wherein the first alkaline solution is a solution of at least one substance selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium bicarbonate, ammonium bicarbonate, sodium carbonate, aqueous ammonia, and potassium carbonate. [8] The method of claim 1, wherein preferably the mass of the surfactant is 0.1% to 2% of the mass of the iron in the first metal solution.

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