A method and system for preparing iron phosphate

Abstract

The present invention provides a method and system for preparing iron phosphate, which comprises first using pre-oxidation treatment to remove manganese ions in lithium iron phosphate lithium extraction tailings, then using alkali treatment to separate carbon elements, then removing calcium and magnesium ions and heavy metal impurities by step-by-step impurity removal, and finally performing oxidation and magnetic screening, and reacting the obtained magnetic iron oxide with phosphoric acid to obtain high-purity iron phosphate. The method for preparing iron phosphate provided by the present invention can achieve the technical effect of obtaining high-purity iron phosphate.

Description

Technical Field

[0001] The present invention relates to the technical field of resource recovery, and particularly relates to a method and a system for preparing iron phosphate. Background Art

[0002] As a kind of lithium battery with high safety, long life and environmental protection, lithium iron phosphate batteries have been widely used in the fields of electric vehicles, energy storage devices and the like in recent years. With the increasing demand for lithium iron phosphate batteries, the recycling of waste batteries has also attracted increasing attention. Lithium iron phosphate, the main cathode material of lithium iron phosphate batteries, often produces waste residues containing phosphorus and iron components (i.e., lithium iron phosphate tailings) after battery retirement or during the production process. The lithium iron phosphate tailings contain rich phosphorus and iron resources. Although they can be recycled to produce battery-grade iron phosphate, technical problems are faced in the process of resource utilization.

[0003] The main components of the lithium-extracted tailings of lithium iron phosphate are phosphorus, iron and carbon, and also contain a small amount of manganese and other heavy metal impurities. These impurities will affect the purity and performance of iron phosphate. At present, in the phosphorus-iron recovery process, it is usually necessary to perform multiple steps of treatment on the tailings, including acid-base treatment, precipitation separation, redox and other operations to remove impurities and improve the product purity. Among them, iron phosphate formed by the reaction of iron hydroxide and phosphoric acid is usually the final target product. However, carbon, manganese and other heavy metal impurities in the tailings will be doped into the crystal structure of iron phosphate during the reaction process, affecting its electrochemical performance and structural stability.

[0004] Therefore, developing a method for efficiently using tailings to prepare high-purity iron phosphate has become an important research direction in this field. Summary of the Invention

[0005] The present invention provides a method for preparing iron phosphate, achieving the technical effect of obtaining iron phosphate with relatively high purity.

[0006] The present invention also provides a preparation system for iron phosphate, achieving the technical effects of simple structure, low operating cost and obtaining iron phosphate with relatively high purity.

[0007] The present invention provides a method for preparing iron phosphate, which includes the following steps:

[0008] Performing pre-oxidation treatment on the lithium-extracted tailings of lithium iron phosphate with an oxidant to obtain pre-oxidized tailings;

[0009] Performing alkali leaching treatment on the pre-oxidized tailings with ammonia water to obtain a carbon-iron mixture and a phosphorus-containing solution;

[0010] Performing acid leaching treatment on the carbon-iron mixture to obtain a carbon mixture and an iron-containing solution;

[0011] Performing reduction treatment on the iron-containing solution to obtain a primary ferrous solution;

[0012] Add a heavy metal removing agent to the primary ferrous solution to obtain a ferrous solution after impurity removal;

[0013] Introduce air into the mixed system including the ferrous solution after impurity removal and ammonia water for oxidation treatment, control the pH of the oxidation treatment system to be 8.0 - 8.5, the ORP to be 200 - 250 mV, and the treatment time of the oxidation treatment to be 30 - 60 min to obtain an iron oxide slurry;

[0014] Perform magnetic screening treatment on the iron oxide slurry to obtain magnetic iron oxide and a remaining slurry;

[0015] React the magnetic iron oxide with phosphoric acid under aeration conditions, and perform calcination treatment on the obtained solid substance to prepare the iron phosphate;

[0016] The oxidant is selected from at least one of ozone and hydrogen peroxide;

[0017] The treatment temperature of the alkali leaching treatment is 70 - 90 °C, and the ammonia water concentration is 5 wt% - 10 wt%;

[0018] The heavy metal removing agent is selected from at least one of sodium fluoride, ammonium fluoride, sodium sulfide, and ammonium sulfide.

[0019] The preparation method of the iron phosphate as described above, wherein reacting the iron oxide slurry with phosphoric acid under aeration conditions specifically includes the following process:

[0020] Under aeration conditions, add phosphoric acid to the magnetic iron oxide to obtain a mixed system of iron phosphate; control the pH of the mixed system of iron phosphate to be 4 - 5, the temperature to be 60 - 70 °C, and keep warm for 20 - 45 min;

[0021] Heat the mixed system of iron phosphate to 70 - 80 °C, keep warm for 20 - 45 min, and then perform solid - liquid separation treatment on the mixed system of iron phosphate to obtain the solid substance.

[0022] The preparation method of the iron phosphate as described above, wherein during the process of reacting the magnetic iron oxide with phosphoric acid under aeration conditions, control the ORP of the mixed system of iron phosphate to be 300 - 350 mV.

[0023] The preparation method of the iron phosphate as described above, wherein before adding the heavy metal removing agent, adding a metal ion complexing agent to the primary ferrous solution is further included;

[0024] The metal ion complexing agent is selected from at least one of citric acid, tartaric acid, and nitrilotriacetic acid; the dosage of the metal ion complexing agent is 1.1 to 1.2 times the sum of the amounts of calcium and magnesium elements in the primary ferrous solution; and / or,

[0025] It also includes adjusting the pH of the primary ferrous solution to 5 to 5.5, and adding polyacrylamide to the primary ferrous solution.

[0026] In the preparation method of iron phosphate as described above, during the reaction of the magnetic iron oxide with phosphoric acid under aeration conditions, it also includes adding a dispersant to the iron phosphate mixing system;

[0027] The dispersant is selected from at least one of polyethylene glycol, polycarboxylate water reducer, and polyacrylamide;

[0028] The dosage of the dispersant is 0.05 to 0.1% of the mass of the magnetic iron oxide.

[0029] In the preparation method of iron phosphate as described above, the calcination treatment temperature is 550 to 700 °C, and the treatment time is 2 to 4 h.

[0030] In the preparation method of iron phosphate as described above, it also includes performing solid-liquid separation on the remaining slurry to obtain an ammonium salt solution;

[0031] Mixing the ammonium salt solution with calcium hydroxide slurry to prepare ammonia water;

[0032] The ammonia water is used as the raw material for the alkali leaching treatment and the oxidation treatment.

[0033] In the preparation method of iron phosphate as described above, it also includes mixing the phosphorus-containing solution with the calcium hydroxide slurry to prepare ammonia water.

[0034] The present invention also provides a preparation system for iron phosphate, which is used to execute any one of the above-mentioned preparation methods of iron phosphate. Among them, the system includes a pre-oxidation reactor, an alkali leaching reactor, an acid leaching reactor, a reduction reactor, a purification reactor, an oxidation reactor, an iron phosphate synthesis reactor, and a calcination reactor that are connected in sequence;

[0035] The oxidation reactor includes a magnetic screening device.

[0036] In the preparation system of iron phosphate as described above, it also includes an ammonia production reactor. The inlets of the ammonia production reactor are respectively connected to the liquid phase outlets of the alkali leaching reactor and the oxidation reactor, and the outlet of the ammonia production reactor is respectively connected to the inlets of the alkali leaching reactor and the oxidation reactor.

[0037] The method for preparing iron phosphate provided by the present invention first uses a pre-oxidation treatment to remove manganese ions in the tailings of lithium iron phosphate extraction, then uses an alkali treatment to separate the carbon element, then removes calcium and magnesium ions and heavy metal impurities through step-by-step impurity removal, and finally reacts the obtained iron hydroxide with phosphoric acid to obtain high-purity iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced one by one below. Obviously, the drawings in the following description are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0039] Figure 1 A schematic diagram of a first system for preparing iron phosphate provided by the present invention;

[0040] Figure 2 This is a schematic diagram of a second system for preparing iron phosphate provided by the present invention.

[0041] Description of reference numerals:

[0042] 100-preoxidation reactor;

[0043] 200-alkali leaching reactor;

[0044] 300-acid leaching reactor;

[0045] 400-reduction reactor;

[0046] 500- impurity removal reactor;

[0047] 600-oxidation reactor;

[0048] 700- Ferric phosphate synthesis reactor;

[0049] 800-calcination reactor;

[0050] 900-Ammonia reactor. DETAILED DESCRIPTION

[0051] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific implementation methods listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0052] At present, the cathode materials made of iron phosphate prepared from lithium iron phosphate tailings as raw materials generally have the defect of poor electrical performance. The inventor believes that the above defect may be caused by the low purity of iron phosphate. Specifically, heavy metal impurities such as aluminum, manganese, copper, and nickel contained in the lithium iron phosphate tailings are difficult to remove during the preparation process of iron phosphate, resulting in low purity of the prepared iron phosphate, which in turn affects the electrical performance of the cathode material. Therefore, if the preparation process can be optimized to remove the impurities in the lithium iron phosphate tailings, the purity of iron phosphate can be improved.

[0053] Based on this, in the first aspect of the present invention, a method for preparing iron phosphate is provided, and the method includes the following steps:

[0054] Pre-oxidize the lithium iron phosphate tailings with an oxidant to obtain pre-oxidized tailings;

[0055] Perform alkali leaching treatment on the pre-oxidized tailings with ammonia water to obtain a carbon-iron mixture and a phosphorus-containing solution;

[0056] Perform acid leaching treatment on the carbon-iron mixture to obtain a carbon mixture and an iron-containing solution;

[0057] Perform reduction treatment on the iron-containing solution to obtain a primary ferrous solution;

[0058] Add a heavy metal removing agent to the primary ferrous solution to obtain a ferrous solution after impurity removal;

[0059] Introduce air into the mixed system including the ferrous solution after impurity removal and ammonia water for oxidation treatment, control the pH of the oxidation treatment system to be 8.0 - 8.5, the ORP to be 200 - 250 mV, and the treatment time of the oxidation treatment to be 30 - 60 min to obtain an iron oxide slurry;

[0060] Perform magnetic screening treatment on the iron oxide slurry to obtain magnetic iron oxide and a remaining slurry;

[0061] React the magnetic iron oxide with phosphoric acid under aeration conditions, and perform calcination treatment on the obtained solid substance to prepare iron phosphate;

[0062] The oxidant is selected from at least one of ozone and hydrogen peroxide;

[0063] The treatment temperature of the alkali leaching treatment is 70 - 90 °C, the treatment pressure is normal pressure, and the ammonia water concentration is 5 wt% - 10 wt%;

[0064] The heavy metal removing agent is selected from at least one of sodium fluoride, ammonium fluoride, sodium sulfide, and ammonium sulfide.

[0065] Lithium iron phosphate tailings are the remaining material obtained after lithium elements are extracted from lithium iron phosphate materials in waste batteries by acid leaching and other methods. The main component is iron phosphate. In the preparation process of the above-mentioned iron phosphate, the pre-oxidation treatment includes adding the lithium iron phosphate tailings to an oxidant for treatment. The oxidant can be selected from at least one of ozone or hydrogen peroxide (concentration 3%-5%). In this process, the soluble manganese element impurities in the lithium iron phosphate tailings are oxidized into manganese dioxide solids, which are convenient for removal by solid-liquid separation in subsequent processes, thereby reducing the soluble impurity metals in the solution.

[0066] The present invention does not limit the specific treatment conditions of the pre-oxidation treatment, as long as the requirement that divalent manganese can be oxidized to manganese dioxide is met. In one embodiment, the treatment temperature of the pre-oxidation treatment is 30-50 ° C, and the treatment time is 30-60 minutes. Controlling the appropriate temperature and time can ensure that the manganese element in the lithium iron phosphate lithium tailings is completely oxidized. After the pre-oxidation treatment is completed, the lithium iron phosphate lithium tailings are converted into pre-oxidation tailings.

[0067] The alkali leaching treatment includes reacting the pre-oxidized tailings with hot ammonia water (ammonia water concentration 5wt%-10wt%) at a temperature of 70-90°C under normal pressure for 1-2 hours. In this process, the pre-oxidized tailings, whose main component is iron phosphate, decompose into iron hydroxide and ammonium dihydrogen phosphate, achieving effective separation of iron and phosphorus elements, and the solid phase substance formed is a carbon-iron mixture including iron hydroxide and carbon element, and the liquid phase substance generated is a phosphorus-containing solution including ammonium dihydrogen phosphate.

[0068] The alkalinity of ammonia water is relatively weak. It can not only effectively decompose iron phosphate, but the remaining ammonia can also escape in the form of ammonia gas, which can be more easily separated and recycled, avoiding the introduction of impurity ions, thereby improving the purity of the iron phosphate product. When sodium hydroxide is used for alkaline leaching, sodium hydroxide will introduce sodium ions during the reaction process. Sodium ions are an impurity that is difficult to completely remove and are easily retained in the subsequent preparation of iron phosphate, affecting the electrochemical properties of the final product.

[0069] The acid leaching treatment includes mixing the carbon-iron mixture with an acid substance, dissolving the iron hydroxide in the carbon-iron mixture to form an iron salt solution, and separating the carbon element and the manganese dioxide formed in the pre-oxidation process from the iron salt solution in a solid form. The present invention is not limited to the specific selection of the acid substance, as long as it can dissolve the iron hydroxide. In one embodiment, the acid substance is selected from at least one of sulfuric acid and hydrochloric acid. The solid-liquid separation treatment is performed on the acid leaching treatment system to obtain a solid phase carbon mixture and a liquid phase iron-containing solution.

[0070] In one embodiment, when the carbon mixture also contains manganese dioxide generated by pre-oxidation treatment, the difference between the density of manganese dioxide (about 5.0 g / cm³) and the density of carbon (about 1.8-2.1 g / cm³) can be used to put the mixture into water. Through gravity sedimentation, the manganese dioxide will settle to the bottom, while the carbon will float on the top due to its lower density, and preliminary separation can be achieved by filtration or layering.

[0071] It is understandable that the carbon mixture can also be cleaned with an acidic substance, and the obtained washing liquid containing iron elements is mixed with the iron-containing solution and then carried out to the next treatment step.

[0072] The reduction treatment specifically includes adding a reducing agent to the iron-containing solution to reduce the trivalent iron to divalent iron to obtain a primary ferrous solution. In the method for preparing ferric phosphate provided by the present invention, the reducing agent is reduced iron powder. During the reduction treatment, the iron powder reduces the trivalent iron to divalent iron, and at the same time, some impurity metals (such as copper, nickel and other metals that are easily reduced) can also be reduced to solid single substances, making it easier to separate from the iron element in subsequent steps.

[0073] Primary ferrous solution often contains heavy metal elements such as copper and nickel. Adding heavy metal impurity remover to the primary ferrous solution can make the heavy metal elements form precipitation and separate from the liquid phase to obtain impurity-free ferrous solution, so that the prepared ferric phosphate has higher purity. Among them, the metal impurity remover can be selected from at least one of sodium fluoride, ammonium fluoride, sodium sulfide and ammonium sulfide.

[0074] Next, air is introduced into the mixed system including the impurity-removing ferrous solution and ammonia water for oxidation treatment, the pH of the oxidation treatment system is controlled to be 8.0-8.5, the ORP is 200-250mV, and the treatment time of the oxidation treatment is 30-60min to obtain an iron oxide slurry. Specifically, ammonia water is added to the impurity-removing ferrous solution, and the ammonia water reacts with the ferrous ions in the impurity-removing ferrous solution to generate ferrous hydroxide. At the same time, air is introduced into the mixed system for oxidation treatment, and the ferrous hydroxide can be at least partially oxidized to ferric hydroxide to obtain a mixture of ferrous hydroxide and ferric hydroxide, which is then dehydrated to ferric oxide. In this process, the pH is kept at 8.0-8.5 and the ORP is kept at 200-250 mV. By controlling a lower ORP and a mild alkaline environment, the generation of ferric oxide can be ensured. If the time is too short, the reaction may be insufficient, and too long a time may lead to over-oxidation, affecting the product structure. Among them, ORP can be measured by an online redox potential analyzer. More specifically, the process can be carried out under heating and stirring conditions, and the heating temperature is 60-80° C. At the above temperature, the mixture of ferrous hydroxide and ferric hydroxide can be dehydrated to ferrosoferric oxide more quickly.

[0075] Subsequently, the iron oxide slurry is subjected to magnetic screening treatment to obtain magnetic iron oxide and residual slurry. The magnetic iron oxide is the iron oxide obtained by oxidation treatment, and the residual slurry is composed of non-magnetic iron compounds and other impurities. The use of magnetic screening treatment can further purify the iron element by utilizing the magnetic properties of iron oxide, so that the obtained iron phosphate has a higher purity.

[0076] It can be understood that, in one embodiment, the magnetic iron oxide is further washed with deionized water to further improve the purity of the magnetic iron oxide, thereby improving the purity of the iron phosphate.

[0077] The magnetic iron oxide obtained by magnetic screening is reacted with phosphoric acid under aeration conditions, and the obtained solid phase material is calcined to obtain iron phosphate. Specifically, since the main component of magnetic iron oxide is ferroferric oxide, it will simultaneously generate divalent iron compounds and trivalent iron compounds when reacting with phosphoric acid. When the above process is carried out under aeration conditions, the oxygen in the air can oxidize the divalent iron compounds into trivalent iron compounds, thereby improving the purity of the obtained iron phosphate.

[0078] After the reaction of magnetic iron oxide and phosphoric acid is completed, the obtained reaction system is subjected to solid-liquid separation, and the obtained solid phase material is iron phosphate. The solid phase material is subjected to calcination treatment to obtain the product iron phosphate. Among them, the calcination treatment can improve the crystallinity and electrochemical properties of iron phosphate.

[0079] It is worth noting that, in order to continue to oxidize the divalent iron that may remain in the iron phosphate, the calcination process can be carried out in an air atmosphere. In addition, in order to further improve the purity of the iron phosphate, before the solid phase material is calcined, deionized water can be used to wash away impurities on the surface of the solid phase material.

[0080] The method for preparing iron phosphate provided by the present invention can achieve the technical effect of preparing iron phosphate with higher purity, because the method first uses pre-oxidation treatment to oxidize manganese ions in the tailings of lithium iron phosphate extraction to form insoluble matter and separate from the iron element; then uses alkali treatment to separate the carbon element, and then removes heavy metal impurities and then oxidizes it, and finally reacts the obtained iron hydroxide with phosphoric acid to obtain high-purity iron phosphate.

[0081] In order to further improve the chemical and physical stability of iron phosphate, in one embodiment, the iron oxide slurry is reacted with phosphoric acid under aeration conditions, specifically including the following process:

[0082] Under aeration conditions, phosphoric acid is added to the magnetic iron oxide to obtain an iron phosphate mixed system; the pH of the iron phosphate mixed system is controlled to be 4-5, the temperature is 60-70°C, and the temperature is kept for 20-45 minutes;

[0083] Heat the iron phosphate mixed system to 70 - 80 °C, keep it warm for 20 - 45 min, and then perform solid-liquid separation on the iron phosphate mixed system to obtain solid-phase substances.

[0084] Controlling the reaction conditions (such as temperature and pH) in stages helps optimize the crystal growth of iron phosphate. In the initial stage, controlling the pH of the mixed system to be 4 - 5 and maintaining it at 60 - 70 °C can make the crystal nucleation process slow and uniform, thereby reducing crystal agglomeration and promoting the formation of more nucleation points. This step ensures that the crystals are more delicate and uniform when generated, laying a good foundation for subsequent growth. The holding time of 20 - 45 minutes allows the initial nucleation of iron phosphate crystals to proceed sufficiently, and the uniformity and size of the crystals are effectively controlled.

[0085] Subsequently, raise the temperature to 70 - 80 °C and then keep it warm for 20 - 45 minutes, which can further promote the growth of iron phosphate crystals. At this stage, the increase in temperature will accelerate ion movement and the reaction rate, enabling the crystals to continue growing without affecting uniform nucleation. The heat preservation treatment in this step can ensure that the crystals further increase in size and have a high crystallinity, improving the physical and chemical stability of the material.

[0086] Therefore, first keep it warm at 60 - 70 °C and then raise the temperature to 70 - 80 °C. This stepwise heating method can effectively control the particle size and morphology of iron phosphate particles. By first keeping it warm at a low temperature and then raising the temperature, excessive agglomeration and non-uniform growth can be effectively avoided. This helps obtain iron phosphate particles with a moderate particle size, ensuring good conductivity and electrochemical activity when used as the cathode material for lithium batteries; in addition, slow heating can also reduce defects inside the crystals and form a more complete crystal structure, so that iron phosphate exhibits higher charge-discharge performance and stability in subsequent applications.

[0087] To further improve the electrochemical performance of iron phosphate, in one embodiment, during the reaction of magnetic iron oxide with phosphoric acid under aeration conditions, control the ORP of the iron phosphate mixed system to be 300 - 350 mV. By controlling the ORP and pH, it can be ensured that ferrous iron is completely oxidized to ferric iron; at the same time, controlling the ORP and pH values helps form multiple nucleation points gradually during the reaction, enabling the subsequent generated iron phosphate to grow on multiple uniform crystal nuclei, reducing the situation of crystal agglomeration, and forming a product with high crystallinity and regular morphology, thereby improving the electrochemical performance of iron phosphate.

[0088] In order to further improve the purity of the prepared iron phosphate, in one embodiment, before adding the heavy metal removing agent, adding a metal ion complexing agent to the primary ferrous solution is further included. The lithium extraction tailings of lithium iron phosphate often contain impurities such as aluminum element, calcium element and magnesium element. These impurities are often difficult to remove in the above-mentioned treatment process and easily enter the iron phosphate product, reducing the purity of iron phosphate. Therefore, before adding the heavy metal removing agent, a metal ion complexing agent can be added to the primary ferrous solution to keep the aluminum element, calcium element and magnesium element in a dissolved state in the system, so that hydroxide precipitates will not be formed due to the addition of ammonia water in the subsequent oxidation treatment. The metal ion complexing agent can specifically be selected from at least one of citric acid, tartaric acid, and nitrilotriacetic acid. The above-mentioned metal ion complexing agent has excellent chelating effects on aluminum element, calcium element and magnesium element, and can keep the aluminum element, calcium element and magnesium element in the solution system without precipitation. At the same time, although the above-mentioned metal ion complexing agent also has a certain complexing effect on Fe²⁺ and Fe³⁺, its chelating constant is much smaller than that for aluminum element, calcium element and magnesium element, so as to effectively separate aluminum element, calcium element and magnesium element from iron element.

[0089] More specifically, the dosage of the metal ion complexing agent can be controlled to be 1.1 - 1.2 times the sum of the amounts of substances of aluminum element, calcium element and magnesium element in the primary ferrous solution. The above dosage can ensure the complete complexation of aluminum element, calcium element and magnesium element, thereby further improving the purity of iron phosphate.

[0090] In addition, before adding the heavy metal removing agent, the pH of the primary ferrous solution can be adjusted to 5 - 5.5, and polyacrylamide can be added to the primary ferrous solution. The lithium extraction tailings of lithium iron phosphate often contain aluminum element impurities. The aluminum element impurities are often difficult to remove in the previous treatment and exist in the primary ferrous solution in the form of aluminum ions after acid leaching treatment. Adjusting the pH of the primary ferrous solution to 5 - 5.5 can make the aluminum ions form aluminum hydroxide gel without causing the precipitation of ferrous ions as ferrous hydroxide. At this time, adding polyacrylamide to the primary ferrous solution can make the aluminum hydroxide gel flocculate and precipitate, thereby separating the aluminum ions from the liquid phase environment. The present invention does not limit the reagent used to adjust the pH. In one embodiment, ammonia water is used to adjust the pH to 5 - 5.5.

[0091] In one embodiment, when it is necessary to remove aluminum ions and complex calcium and magnesium ions at the same time, the pH of the primary ferrous solution can be first adjusted to 5 - 5.5, and polyacrylamide can be added to the primary ferrous solution to precipitate the aluminum ions; then a metal ion complexing agent can be added to the primary ferrous solution. The above sequence can avoid the aluminum ions from being unable to form a precipitate due to complexation.

[0092] In a specific embodiment, during the reaction of magnetic iron oxide with phosphoric acid under aeration conditions, a dispersant is further added to the iron phosphate mixed system; the dispersant is selected from at least one of polyethylene glycol, polycarboxylate water reducer, and polyacrylamide; the dosage of the dispersant is 0.05 - 0.1% of the mass of the magnetic iron oxide. The dispersant can achieve a high degree of dispersion of iron phosphate through the principle of reducing viscosity by particle repulsion, making the prepared iron phosphate have uniform particle size and thus better electrical properties.

[0093] In one embodiment, the treatment temperature of the calcination treatment can be controlled at 550 - 700 °C, and the treatment time is 2 - 4 h. The above treatment conditions can improve its crystallinity and purity, optimize the crystal structure, make the material denser and reduce impurities. High-temperature sintering promotes crystal growth, improves crystallinity and physical strength, reduces structural defects, and enhances the stability and conductivity of the material. The heat preservation time ensures sufficient crystal growth, significantly improving the electrochemical performance and cycle life of the material. In addition, high temperature helps to form a stable olivine structure, making iron phosphate suitable for use as the cathode material of lithium batteries, with good electrochemical performance and stability.

[0094] In one embodiment, it further includes performing solid-liquid separation treatment on the remaining slurry to obtain an ammonium salt solution;

[0095] Mix the ammonium salt solution with calcium hydroxide slurry to prepare ammonia water;

[0096] The ammonia water is used as a raw material for alkali leaching treatment and oxidation treatment.

[0097] The remaining slurry is the residue after separating magnetic iron oxide from the iron oxide slurry. Performing solid-liquid separation treatment on it can obtain a liquid-phase product containing ammonium salts. The type of ammonium salt is related to the type of acid used in the previous acid leaching treatment. For example, when sulfuric acid is used in the acid leaching treatment, the ammonium salt is ammonium sulfate. Mixing the ammonium salt solution with calcium hydroxide slurry to prepare ammonia water. Specifically, the ammonia obtained by mixing the ammonium salt solution with calcium hydroxide slurry dissolved in water can be used to prepare ammonia water, and the prepared ammonia water can be used as a raw material for alkali leaching treatment and oxidation treatment. The above process realizes the recycling of ammonia and ammonium salts, making the preparation method of iron phosphate provided by the present invention have lower production costs.

[0098] More specifically, in one embodiment, the calcium hydroxide slurry can use carbide sludge remaining from industrial acetylene production as the calcium hydroxide slurry. Carbide sludge is the residue after the reaction of calcium carbide with water to produce acetylene, and its main component is calcium hydroxide. Using carbide sludge as the calcium hydroxide slurry can further reduce the production cost of the preparation method of iron phosphate provided by the present invention.

[0099] In another embodiment, ammonia water can also be prepared by the following method: mixing a phosphorus-containing solution with a calcium hydroxide slurry to prepare ammonia water. Wherein, the phosphorus-containing solution is prepared by alkali leaching of pre-oxidized tailings, and the main component is ammonium dihydrogen phosphate. Mixing the phosphorus-containing solution with calcium hydroxide slurry can also prepare ammonia water, which can be used as a raw material for alkali leaching and oxidation treatment. The above process further realizes the recycling of ammonia and ammonium salts, so that the preparation method of iron phosphate provided by the present invention has a lower production cost.

[0100] A second aspect of the present invention provides a system for preparing iron phosphate, which is used to perform any one of the methods for preparing iron phosphate provided in the first aspect of the present invention. Figure 1 A schematic diagram of a preparation system of the first iron phosphate provided by the present invention is shown in FIG. Figure 1 As shown, the system includes 100-pre-oxidation reactor, 200-alkali leaching reactor, 300-acid leaching reactor, 400-reduction reactor, 500-impurity removal reactor, 600-oxidation reactor, 700-ferric phosphate synthesis reactor, and 800-calcination reactor which are connected in sequence.

[0101] Among them, the 100-pre-oxidation reactor is used to perform pre-oxidation treatment. It can be understood that in addition to accommodating the reactants for reaction, the 100-pre-oxidation reactor also has a solid-liquid separation function. Specifically, in the pre-oxidation treatment, the lithium iron phosphate tailings are mixed with an oxidant to oxidize the manganese element in the lithium iron phosphate tailings into manganese dioxide; then, the reaction system is subjected to solid-liquid separation, and the solid phase obtained is the pre-oxidation tailings, which enters the 200-alkali leaching reactor for alkali leaching treatment.

[0102] 200-Alkali leaching reactor is used to perform alkaline leaching treatment. It is understood that the alkaline leaching processor, in addition to being used to accommodate reactants for reaction, also has a solid-liquid separation function. Specifically, the pre-oxidation tailings react with ammonia water in the 200-alkali leaching reactor to decompose into a carbon-iron mixture whose main component is iron hydroxide and a phosphorus-containing solution whose main component is diammonium phosphate. The carbon-iron mixture continues to enter the 300-acid leaching reactor for acid leaching treatment.

[0103] 300-Acid leaching reactor is used to perform acid leaching treatment. It is understandable that the acid leaching processor, in addition to being used to accommodate reactants for reaction, also has a solid-liquid separation function. Specifically, the carbon-iron mixture reacts with an acid substance (e.g., sulfuric acid) in the 300-acid leaching reactor, and the iron hydroxide in the carbon-iron mixture dissolves into iron ions, and the carbon element becomes a precipitate. Decomposed into a main component of iron hydroxide, then the reaction system is solid-liquid separated, and the obtained liquid phase substance is an iron-containing solution, which enters the 400-reduction reactor for reduction treatment.

[0104] The 400-Reduction Reactor is used to perform reduction treatment. It can be understood that in addition to accommodating reactants for reaction, the reduction processor also has a solid-liquid separation function. Specifically, the 400-Reduction Reactor accommodates the reaction of an iron-containing solution with a reducing agent. Specifically, the reducing agent (e.g., reduced iron powder) reacts with the iron ions included in the iron-containing solution, generating ferrous ions while undergoing a displacement reaction with the metal ions (e.g., copper ions) in the iron-containing solution, separating the copper element from the iron element. Subsequently, solid-liquid separation is performed on the reaction system, and the resulting liquid-phase substance is the primary ferrous solution, which enters the 500-Impurity Removal Reactor.

[0105] The 500-Impurity Removal Reactor is used to accommodate the reaction of the primary ferrous solution with a heavy metal impurity remover to remove heavy metal elements from the primary ferrous solution. It can be understood that in addition to accommodating reactants for reaction, the impurity removal processor also has a solid-liquid separation function. Specifically, the 500-Impurity Removal Reactor reacts with the heavy metal impurity remover, and the heavy metal forms a solid-phase precipitate, separating from the iron element. Subsequently, solid-liquid separation is performed on the reaction system, and the resulting liquid-phase substance is the impurity-removed ferrous solution, which enters the 600-Oxidation Reactor.

[0106] The 600-Oxidation Reactor is used to accommodate the reaction of the impurity-removed ferrous solution with ammonia water and oxygen from aeration to obtain an iron oxide slurry. Specifically, under a heated state, after the impurity-removed ferrous solution enters the 600-Oxidation Reactor, it is mixed with ammonia water to generate ferrous hydroxide. Subsequently, air is introduced into the 600-Oxidation Reactor to oxidize at least part of the ferrous hydroxide to ferric hydroxide, and then dehydrated to magnetite to obtain an iron oxide slurry, which enters the magnetic screening device.

[0107] Furthermore, the 600-Oxidation Reactor includes a magnetic screening device. The magnetic screening device is used to adsorb the magnetic iron oxide in the iron oxide slurry and put the magnetic iron oxide into the 700-Iron Phosphate Synthesis Reactor.

[0108] In addition, in addition to accommodating reactants for reaction, the 600-Oxidation Reactor also has a solid-liquid separation function. Specifically, after the magnetic screening device separates the magnetic iron oxide from the iron oxide slurry, the 600-Oxidation Reactor performs solid-liquid separation on the remaining slurry, and the resulting liquid-phase product is the ammonium salt solution, which can be reserved for the production of ammonia water. The magnetic iron oxide separated by the magnetic screening device enters the 700-Iron Phosphate Synthesis Reactor.

[0109] The 700 - iron phosphate synthesis reactor is used to accommodate the reaction of magnetic iron oxide with phosphoric acid under aeration conditions to produce iron phosphate. It can be understood that the 700 - iron phosphate synthesis reactor not only accommodates the reactants for reaction but also has a solid - liquid separation function. Specifically, after the iron oxide slurry and phosphoric acid react in the 700 - iron phosphate synthesis reactor to produce iron phosphate, the reaction system is subjected to solid - liquid separation, and the solid phase obtained is iron phosphate, which enters the 800 - calcination reactor for calcination treatment.

[0110] The iron phosphate preparation system provided by the present invention realizes the preparation of high - purity iron phosphate with fewer devices, and has the characteristics of simple structure and low operating cost.

[0111] Figure 2 This is the schematic diagram of the second iron phosphate preparation system provided by the present invention. As Figure 2 shown, the iron phosphate preparation system provided by the present invention further includes a 900 - ammonia production reactor. The inlets of the 900 - ammonia production reactor are respectively connected to the liquid phase outlet of the 200 - alkali leaching reactor and the liquid phase outlet of the 600 - oxidation reactor, and the outlets of the 900 - ammonia production reactor are respectively connected to the inlet of the 200 - alkali leaching reactor and the inlet of the 600 - oxidation reactor.

[0112] The 900 - ammonia production reactor is used to produce ammonia water. Among them, ammonia water can be prepared by any one of the following two methods:

[0113] First, after the magnetic screening device separates the magnetic iron oxide in the iron oxide slurry, the 600 - oxidation reactor performs solid - liquid separation on the remaining slurry. The liquid phase product obtained is the ammonium salt solution, which enters the 900 - ammonia production reactor from the liquid phase outlet of the 600 - oxidation reactor. Calcium hydroxide slurry is added to the 900 - ammonia production reactor to prepare ammonia water. Specifically, the ammonium salt reacts with the calcium hydroxide slurry to generate ammonia gas, and the generated ammonia gas is dissolved in water to prepare ammonia water.

[0114] Second, in the 200 - alkali leaching reactor, ammonia water is used to perform alkali leaching treatment on the pre - oxidized tail slag. After obtaining the carbon - iron mixture and the phosphorus - containing solution, the phosphorus - containing solution mainly composed of ammonium dihydrogen phosphate enters the 900 - ammonia production reactor, and calcium hydroxide slurry is added to prepare ammonia water. Specifically, the phosphorus - containing solution reacts with the calcium hydroxide slurry to generate ammonia gas, and the generated ammonia gas is dissolved in water to prepare ammonia water.

[0115] The setting of the 900 - ammonia production reactor realizes the recycling and recovery of ammonia. At the same time, using industrial waste as raw materials makes the characteristic of low operating cost of the iron phosphate preparation system provided by the present invention more prominent.

[0116] The above-mentioned equipment with solid-liquid separation function can perform solid-liquid separation by means of filter pressing, and the solid matter obtained by solid-liquid separation can be washed with deionized water to obtain higher purity.

[0117] Hereinafter, the method for preparing the iron phosphate provided by the present invention will be described in detail through specific examples.

[0118] Example 1

[0119] This embodiment uses lithium iron phosphate tailings to prepare iron phosphate, which specifically includes the following steps:

[0120] Pre-oxidation treatment

[0121] The tailings from lithium iron phosphate extraction were used as raw materials, and the tailings were put into a 100-pre-oxidation reactor, and a 4wt% hydrogen peroxide solution was added thereto, and the reaction was stirred at 40°C for 45 minutes. Through this pre-oxidation treatment, the divalent manganese in the tailings was oxidized to manganese dioxide, forming an insoluble precipitate, reducing the content of soluble impurities in the solution.

[0122] Hot ammonia alkaline leaching

[0123] The tailings after pre-oxidation enter the 200-alkali leaching reactor and react with 8wt% ammonia water at 85°C and normal pressure for 1.5 hours. This step effectively separates iron and phosphorus, and the obtained reaction system is subjected to solid-liquid separation to obtain a carbon-iron mixture that generates iron hydroxide and carbon element, as well as a phosphorus-containing solution of diammonium phosphate. Among them, the carbon-iron mixture enters the 300-acid leaching reactor, and the phosphorus-containing solution enters the 900-ammonia production reactor.

[0124] Acid leaching

[0125] The carbon-iron mixture is added to a 300-acid leaching reactor, mixed with 3% sulfuric acid, and stirred at room temperature for 1 hour to dissolve the iron hydroxide in the carbon-iron mixture into an iron salt solution, and the carbon and manganese dioxide are precipitated in solid form. Subsequently, the carbon and manganese dioxide are separated by solid-liquid separation to obtain a pure iron salt solution.

[0126] Restore Process

[0127] Add the iron salt solution to a 400°C reduction reactor, add reduced iron powder thereto, stir at room temperature for 30 minutes, reduce the trivalent iron in the iron salt solution to divalent iron, and perform solid-liquid separation to obtain a liquid primary ferrous solution. This step can also reduce impurities such as copper and nickel to a solid state, thereby facilitating solid-liquid separation.

[0128] Impurity removal

[0129] Add the primary ferrous solution to the 500-impurity removal reactor, adjust the pH of the primary ferrous solution to 5.5 using ammonia water, and then add polyacrylamide to the primary ferrous solution to precipitate aluminum hydroxide gel. Subsequently, add citric acid to the primary ferrous solution, and the dosage of citric acid is 1.2 times the sum of the amounts of calcium and magnesium elements in the primary ferrous solution. Then add sodium fluoride to the primary ferrous solution with a concentration of 0.15%, and react for 30 minutes under stirring to form insoluble precipitates of heavy metal impurities such as copper and zinc. Subsequently, filter to remove the impurities generated in the above steps to obtain the impurity-removed ferrous solution.

[0130] Oxidation treatment

[0131] Add the impurity-removed ferrous solution to the 600-oxidation reactor, gradually add ammonia water and introduce air into it. Control the pH of the reaction system to 8.2 by controlling the amount of ammonia water added, and control the ORP of the reaction system to 230 mV by controlling the aeration rate. React for 50 minutes to partially oxidize ferrous hydroxide to ferric hydroxide, and then dehydrate to form magnetite, obtaining an iron oxide slurry. Perform magnetic screening treatment on the iron oxide slurry to obtain magnetic iron oxide and the remaining slurry.

[0132] Generate iron phosphate

[0133] Add the magnetic iron oxide to the 700-iron phosphate synthesis reactor. Under aeration conditions, mix the magnetic iron oxide with phosphoric acid aqueous solution, control the pH of the reaction system to 4.5 and the temperature to 65 °C, and keep warm for 30 minutes to ensure the preliminary nucleation of iron phosphate crystals. Subsequently, raise the temperature of the system to 75 °C and keep warm for 30 minutes to promote the further growth of iron phosphate crystals. Finally, obtain the iron phosphate solid phase substance through solid-liquid separation.

[0134] Calcination treatment

[0135] Add the obtained iron phosphate solid phase substance to the 800-calcination reactor, and calcine in an air atmosphere at 700 °C for 3 hours to improve its crystallinity and purity, and finally obtain a battery-grade iron phosphate product.

[0136] Recovery and recycling of ammonia

[0137] Perform solid-liquid separation on the remaining slurry to obtain a liquid phase ammonium salt solution;

[0138] Add the ammonium salt solution to the 900-ammonia production reactor, mix it with calcium hydroxide slurry to produce ammonia water, dissolve the generated ammonia gas in water to obtain ammonia water, which is used as a raw material for alkali leaching treatment and oxidation treatment.

[0139] In addition, it also includes using ammonia water to perform alkali leaching treatment on the pre-oxidized tailings in a 200-alkali leaching reactor. After obtaining a carbon-iron mixture and a phosphorus-containing solution, the phosphorus-containing solution mainly composed of ammonium dihydrogen phosphate enters a 900-ammonia production reactor, adding calcium hydroxide slurry, dissolving the generated ammonia gas in water, and preparing ammonia water as a raw material for alkali leaching treatment and oxidation treatment.

[0140] Test Example

[0141] The impurity element contents and the purity of iron phosphate in the lithium-extracted tailings of lithium iron phosphate were respectively detected using the ICP-OES method. The results are shown in Table 1:

[0142] Table 1 Comparison of impurity contents of samples

[0143]

[0144] Among them, the lithium-extracted tailings of lithium iron phosphate contain 23.9 wt% of iron element and 12.4 wt% of phosphorus element.

[0145] As can be seen from Table 1, the preparation method of iron phosphate provided by the present invention can effectively remove the impurity elements of manganese, copper, aluminum, calcium, and magnesium in the lithium-extracted tailings of lithium iron phosphate. At the same time, the prepared iron phosphate has a high purity.

[0146] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments and comparative examples, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing iron phosphate, characterized in that, It includes the following steps: The lithium-extracted tailings of lithium iron phosphate are pre-oxidized with an oxidant to obtain pre-oxidized tailings, wherein the soluble manganese element impurities in the lithium-extracted tailings of lithium iron phosphate are oxidized to manganese dioxide solids; The pre-oxidized tailings are subjected to alkali leaching treatment with ammonia water to obtain a carbon-iron mixture and a phosphorus-containing solution to achieve the separation of iron and phosphorus. The carbon-iron mixture includes iron hydroxide and carbon; The carbon-iron mixture is subjected to acid leaching treatment, and the iron hydroxide is dissolved to form an iron salt, obtaining a carbon mixture and an iron-containing solution; The iron-containing solution is subjected to reduction treatment to obtain a primary ferrous solution; A heavy metal removing agent is added to the primary ferrous solution to obtain a ferrous solution after impurity removal; Air is introduced into a mixed system including the ferrous solution after impurity removal and ammonia water for oxidation treatment, controlling the pH of the oxidation treatment system to be 8.0 - 8.5 and the ORP to be 200 - 250 mV. The treatment time of the oxidation treatment is 30 - 60 min to obtain an iron oxide slurry; the iron oxide slurry includes magnetite; The iron oxide slurry is subjected to magnetic screening treatment to obtain magnetic iron oxide and a remaining slurry; The magnetic iron oxide reacts with phosphoric acid under aeration conditions, and the obtained solid phase substance is calcined to prepare the iron phosphate; The oxidant is selected from at least one of ozone and hydrogen peroxide; The treatment temperature of the alkali leaching treatment is 70 - 90 °C, and the ammonia water concentration is 5 wt% - 10 wt%; The heavy metal removing agent is selected from at least one of sodium fluoride, ammonium fluoride, sodium sulfide, and ammonium sulfide; Before adding the heavy metal removing agent, a metal ion complexing agent is further added to the primary ferrous solution; The metal ion complexing agent is selected from at least one of citric acid, tartaric acid, and nitrilotriacetic acid; the dosage of the metal ion complexing agent is 1.1 - 1.2 times the sum of the amounts of calcium and magnesium elements in the primary ferrous solution; and / or, It also includes adjusting the pH of the primary ferrous solution to 5 - 5.5 and adding polyacrylamide to the primary ferrous solution.

2. The preparation method of iron phosphate according to claim 1, characterized in that, The reaction of the iron oxide slurry with phosphoric acid under aeration conditions specifically includes the following process: Under aeration conditions, phosphoric acid is added to the magnetic iron oxide to obtain a ferric phosphate mixed system; controlling the pH of the ferric phosphate mixed system to be 4 - 5 and the temperature to be 60 - 70 °C, and keeping warm for 20 - 45 min; The ferric phosphate mixed system is heated to 70 - 80 °C and kept warm for 20 - 45 min, and then the ferric phosphate mixed system is subjected to solid-liquid separation treatment to obtain the solid phase substance.

3. The preparation method of iron phosphate according to claim 2, characterized in that, During the reaction of the magnetic iron oxide with phosphoric acid under aeration conditions, the ORP of the ferric phosphate mixed system is controlled to be 300 - 350 mV.

4. The preparation method of iron phosphate according to claim 1, wherein, During the reaction of the magnetic iron oxide with phosphoric acid under aeration conditions, a dispersant is further added to the ferric phosphate mixed system; The dispersant is selected from at least one of polyethylene glycol, polycarboxylate water reducer, and polyacrylamide; The dosage of the dispersant is 0.05 - 0.1% of the mass of the magnetic iron oxide.

5. The preparation method of iron phosphate according to claim 1, characterized in that, The treatment temperature of the calcination treatment is 550-700 °C, and the treatment time is 2-4 h.

6. The preparation method of iron phosphate according to claim 1, characterized in that, It also includes solid-liquid separation treatment of the remaining slurry to obtain an ammonium salt solution; Mix the ammonium salt solution with calcium hydroxide slurry to prepare ammonia water; The ammonia water is used as the raw material for the alkali leaching treatment and the oxidation treatment.

7. The preparation method of iron phosphate according to claim 6, characterized in that, It also includes mixing the phosphorus-containing solution with the calcium hydroxide slurry to prepare ammonia water.

8. A preparation system for ferric phosphate for performing the preparation method of ferric phosphate according to any one of claims 1-7, characterized in that, It includes a pre-oxidation reactor, an alkali leaching reactor, an acid leaching reactor, a reduction reactor, a purification reactor, an oxidation reactor, a ferric phosphate synthesis reactor, and a calcination reactor connected in sequence; The oxidation reactor includes a magnetic screening device.

9. The preparation system of iron phosphate according to claim 8, characterized in that, It also includes an ammonia production reactor. The inlet of the ammonia production reactor is respectively connected to the liquid phase outlet of the alkali leaching reactor and the liquid phase outlet of the oxidation reactor, and the outlet of the ammonia production reactor is respectively connected to the inlet of the alkali leaching reactor and the inlet of the oxidation reactor.

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