Preparation method and system of iron phosphate

Abstract

The invention provides a preparation method and system of iron phosphate, and the method comprises the following steps: firstly removing manganese ions in lithium iron phosphate lithium extraction tailings by using pre-oxidation treatment, then separating elemental carbon by using alkali treatment, then removing calcium and magnesium ions and heavy metal impurities by step-by-step impurity removal, and finally carrying out oxidation and magnetic screening to obtain the iron phosphate. And reacting the obtained magnetic iron oxide with phosphoric acid to obtain high-purity iron phosphate. The preparation method of iron phosphate provided by the invention can realize the technical effect of preparing iron phosphate with relatively high purity.

Description

Technical Field

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

[0002] As a lithium battery with high safety, long life and environmental protection, lithium iron phosphate battery has been widely used in electric vehicles, energy storage devices and other fields in recent years. With the increase in demand for lithium iron phosphate batteries, the recycling of waste batteries has also received increasing attention. Lithium iron phosphate, the main positive electrode material of lithium iron phosphate batteries, often produces waste slag containing phosphorus and iron components (i.e., lithium iron phosphate tailings) after the battery is retired or during the production process. Lithium iron phosphate tailings contain rich phosphorus and iron resources. Although they can be recycled to produce battery-grade iron phosphate, they face technical difficulties in the process of resource utilization.

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

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

[0005] The present invention provides a method for preparing ferric phosphate, which achieves the technical effect of preparing ferric phosphate with higher purity.

[0006] The present invention also provides a system for preparing ferric phosphate, which has the technical effect of simple structure, low operating cost and capable of preparing ferric phosphate with higher purity.

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

[0008] Using an oxidant to pre-oxidize the lithium iron phosphate tailings to obtain pre-oxidized tailings;

[0009] Using ammonia water to perform alkaline leaching on the pre-oxidation tailings to obtain a carbon-iron mixture and a phosphorus-containing solution;

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

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

[0012] adding a heavy metal impurity remover to the primary ferrous solution to obtain an impurity-removed ferrous solution;

[0013] 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 controlled to be 200-250 mV, and the treatment time of the oxidation treatment is 30-60 min to obtain an iron oxide slurry;

[0014] Performing magnetic screening on the iron oxide slurry to obtain magnetic iron oxide and residual slurry;

[0015] The magnetic iron oxide is reacted with phosphoric acid under aeration conditions, and the obtained solid phase material is calcined to obtain the iron phosphate;

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

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

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

[0019] The method for preparing iron phosphate as described above, wherein the iron oxide slurry is reacted with phosphoric acid under aeration conditions, specifically comprises the following process:

[0020] Under aeration conditions, adding phosphoric acid to the magnetic iron oxide to obtain an iron phosphate mixed system; controlling the pH of the iron phosphate mixed system to 4-5 and the temperature to 60-70° C., and keeping the temperature for 20-45 minutes;

[0021] The ferric phosphate mixed system is heated to 70-80° C. and kept warm for 20-45 minutes, and then the ferric phosphate mixed system is subjected to solid-liquid separation treatment to obtain the solid phase substance.

[0022] In the method for preparing iron phosphate as described above, during the reaction of the magnetic iron oxide with phosphoric acid under aeration conditions, the ORP of the iron phosphate mixed system is controlled to be 300-350 mV.

[0023] The method for preparing ferric phosphate as described above, wherein before adding the heavy metal impurity remover, it also includes adding a metal ion complexing agent to the primary ferrous solution;

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

[0025] The method also includes adjusting the pH of the primary ferrous solution to 5-5.5 and adding polyacrylamide to the primary ferrous solution.

[0026] The method for preparing iron phosphate as described above, wherein the process of reacting the magnetic iron oxide with phosphoric acid under aeration conditions further comprises adding a dispersant to the iron phosphate mixed system;

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

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

[0029] In the method for preparing iron phosphate as described above, the treatment temperature of the calcination treatment is 550-700° C., and the treatment time is 2-4 hours.

[0030] The method for preparing ferric phosphate as described above, wherein the method further comprises subjecting the remaining slurry to solid-liquid separation to obtain an ammonium salt solution;

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

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

[0033] The method for preparing ferric phosphate as described above further comprises mixing the phosphorus-containing solution with the calcium hydroxide slurry to produce ammonia water.

[0034] The present invention also provides a system for preparing iron phosphate, which is used to perform any of the above-mentioned methods for preparing iron phosphate, wherein the system comprises a pre-oxidation reactor, an alkali leaching reactor, an acid leaching reactor, a reduction reactor, an impurity removal reactor, an oxidation reactor, an iron phosphate synthesis reactor, and a calcination reactor which are connected in sequence;

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

[0036] The system for preparing iron phosphate as described above, wherein, further comprises an ammonia production reactor, wherein 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.

[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 positive electrode materials made of iron phosphate using lithium iron phosphate tailings as raw materials generally have the defect of poor electrical performance. The inventors believe that the above defects may be caused by the low purity of iron phosphate. Specifically, the heavy metal impurities such as aluminum, manganese, copper, and nickel contained in lithium iron phosphate tailings are difficult to remove during the preparation process of iron phosphate, resulting in low purity of the obtained iron phosphate, which in turn affects the electrical properties of the positive electrode materials. Therefore, if the impurities in the lithium iron phosphate tailings can be removed by optimizing the preparation process, the purity of iron phosphate can be improved.

[0053] Based on this, the first aspect of the present invention provides a method for preparing iron phosphate, the method comprising the following steps:

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

[0055] Using ammonia water to alkaline-leach the pre-oxidation tailings to obtain a carbon-iron mixture and a phosphorus-containing solution;

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

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

[0058] adding a heavy metal impurity remover to the primary ferrous solution to obtain an impurity-removed ferrous solution;

[0059] Air is introduced into a mixed system including a decontaminated 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 controlled to be 200-250 mV, and the treatment time of the oxidation treatment is 30-60 minutes to obtain an iron oxide slurry;

[0060] Performing magnetic screening on the iron oxide slurry to obtain magnetic iron oxide and residual slurry;

[0061] The magnetic iron oxide is reacted with phosphoric acid under aeration conditions, and the obtained solid phase material is calcined to obtain iron phosphate;

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

[0063] The treatment temperature of the alkaline leaching treatment is 70~90℃, the treatment pressure is normal pressure, and the ammonia concentration is 5wt%~10wt%;

[0064] The heavy metal impurity remover 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 is understandable 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] The ferric phosphate mixed system is heated to 70-80° C. and kept warm for 20-45 minutes, and then the ferric phosphate mixed system is subjected to solid-liquid separation to obtain a solid phase substance.

[0084] Controlling 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 4-5 and maintaining it at 60-70°C can make the nucleation process of the crystals 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 during generation, laying a good foundation for subsequent growth. The holding time of 20-45 minutes allows the initial nucleation of iron phosphate crystals to proceed fully, and the uniformity and size of the crystals are effectively controlled.

[0085] Subsequently, the temperature is raised to 70-80°C and then kept warm for 20-45 minutes to further promote the growth of iron phosphate crystals. At this stage, the increase in temperature will accelerate the movement of ions and the reaction rate, allowing the crystals to continue to grow without affecting uniform nucleation. This step of heat preservation can ensure that the crystals are further enlarged and have a higher degree of crystallinity, improving the physical and chemical stability of the material.

[0086] Therefore, the temperature is kept at 60-70℃ first, and then raised to 70-80℃. This step-by-step heating method can effectively control the particle size and morphology of iron phosphate particles. By keeping the temperature at a low temperature first and then raising the temperature, excessive agglomeration and uneven growth can be effectively avoided. This helps to obtain iron phosphate particles with a moderate particle size, ensuring good conductivity and electrochemical activity when used in lithium battery positive electrode materials; in addition, slow heating can also reduce defects inside the crystal and form a more complete crystal structure, so that the iron phosphate will show higher charge and discharge performance and stability in subsequent applications.

[0087] In order to further improve the electrochemical performance of iron phosphate, in one embodiment, the ORP of the iron phosphate mixed system can be controlled to be 300-350mV during the reaction of magnetic iron oxide with phosphoric acid under aeration conditions. By controlling the ORP and pH, it can be ensured that the divalent iron is completely oxidized to trivalent iron; at the same time, controlling the ORP and pH value helps to gradually form multiple nucleation points during the reaction process, so that the subsequently generated iron phosphate can grow on multiple uniform crystal nuclei, reduce crystal agglomeration, and form 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 obtained ferric phosphate, in one embodiment, before adding the heavy metal impurity remover, a metal ion complexing agent is also added to the primary ferrous solution. The lithium iron phosphate tailings often contain aluminum, calcium and magnesium impurities, which are often difficult to remove in the aforementioned treatment process and easily enter the ferrous phosphate product, reducing the purity of the ferric phosphate. Therefore, before adding the heavy metal impurity remover, a metal ion complexing agent can be added to the primary ferrous solution to keep the aluminum, calcium and magnesium elements dissolved in the system, so that hydroxide precipitation will not be generated in the subsequent oxidation treatment due to the addition of ammonia water in the system. The metal ion complexing agent can be specifically selected from at least one of citric acid, tartaric acid, and nitrilotriacetic acid. The above metal ion complexing agent has excellent chelating effect on aluminum, calcium and magnesium, and can keep aluminum, calcium and magnesium in the solution system without precipitation; at the same time, although the above metal ion complexing agent also has a certain complexing effect on Fe²⁺ and Fe³⁺, its chelating constant is much smaller than the chelating constant for aluminum, calcium and magnesium, thereby realizing the effective separation of aluminum, calcium and magnesium from iron.

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

[0090] In addition, before adding the heavy metal impurity remover, 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 iron phosphate tailings often contain aluminum impurities, which 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. Adjusting the pH of the primary ferrous solution to 5~5.5 can make the aluminum ions form aluminum hydroxide gel without causing the ferrous ions to generate ferrous hydroxide precipitation. At this time, adding polyacrylamide to the primary ferrous solution can cause the aluminum hydroxide gel to 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 aluminum ions need to be removed and calcium and magnesium ions need to be complexed at the same time, the pH of the primary ferrous solution can be adjusted to 5-5.5, and polyacrylamide can be added to the primary ferrous solution to precipitate aluminum ions; and then a metal ion complexing agent can be added to the primary ferrous solution. The above sequence can prevent aluminum ions from being complexed and unable to form precipitation.

[0092] In a specific embodiment, the process of reacting the magnetic iron oxide with phosphoric acid under aeration conditions further includes adding a dispersant to the iron phosphate mixed system; the dispersant is selected from at least one of polyethylene glycol, polycarboxylic acid water reducer, and polyacrylamide; the amount of the dispersant added is 0.05-0.1% of the mass of the magnetic iron oxide. The dispersant can achieve high dispersion of the iron phosphate by reducing the viscosity through the particle repulsion effect, so that the obtained iron phosphate has a uniform particle size, thereby having better electrical properties.

[0093] In one embodiment, the treatment temperature of the calcination treatment can be controlled to be 550~700℃, and the treatment time is 2~4h. 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 insulation time ensures sufficient crystal growth, which significantly improves the electrochemical properties and cycle life of the material. In addition, high temperature helps to form a stable olivine structure, making iron phosphate suitable for positive electrode materials for lithium batteries, with good electrochemical properties and stability.

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

[0095] Mixing an ammonium salt solution with a calcium hydroxide slurry to produce aqueous ammonia;

[0096] Ammonia water is used as raw material for alkaline leaching and oxidation treatment.

[0097] The residual slurry is the residue after the magnetic iron oxide is separated from the iron oxide slurry. A liquid phase product including an ammonium salt can be obtained by subjecting it to a solid-liquid separation treatment. The type of ammonium salt is related to the type of acid substance 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. The ammonium salt solution is mixed with the calcium hydroxide slurry to obtain ammonia water. Specifically, the ammonia obtained by mixing the ammonium salt solution with the calcium hydroxide slurry is dissolved in water to obtain ammonia water. The obtained ammonia water can be used as a raw material for alkali leaching and oxidation treatment. The above process realizes the recycling of ammonia and ammonium salt, so that the preparation method of iron phosphate provided by the present invention has a lower production cost.

[0098] More specifically, in one embodiment, the calcium hydroxide slurry can use the calcium carbide mud left over from the industrial production of acetylene as the calcium hydroxide slurry. Calcium carbide mud is the residue of acetylene produced by the reaction of calcium carbide and water, and its main component is calcium hydroxide. Using calcium carbide mud as calcium hydroxide slurry can further reduce the production cost of the preparation method of ferric 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] 400-reduction reactor is used to perform reduction treatment. It is understandable that the reduction processor, in addition to being used to accommodate reactants for reaction, also has a solid-liquid separation function. Specifically, 400-reduction reactor accommodates the iron-containing solution and reacts with the reducing agent. Specifically, the reducing agent (e.g., reduced iron powder) reacts with the iron ions included in the iron-containing solution to generate ferrous ions, and at the same time, a replacement reaction occurs with the metal ions (e.g., copper ions) in the iron-containing solution to separate the copper element from the iron element. Subsequently, the reaction system is subjected to solid-liquid separation, and the resulting liquid phase substance is the primary ferrous solution, which enters the 500-impurity removal reactor.

[0105] 500-removal of impurities reactor is used to accommodate the reaction of primary ferrous solution and heavy metal impurity remover, and removes the heavy metal elements in the primary ferrous solution. It is understandable that the impurity removal processor, in addition to being used to accommodate reactants for reaction, also has a solid-liquid separation function. Specifically, 500-removal of impurities reactor reacts with heavy metal impurity remover, and heavy metals generate solid phase precipitation and are separated from iron elements. Subsequently, the reaction system is subjected to solid-liquid separation, and the liquid phase material obtained is the impurity removal ferrous solution, which enters 600-oxidation reactor.

[0106] The 600-oxidation reactor is used to contain the impurity-removed ferrous solution and react with ammonia water and oxygen from aeration to obtain iron oxide slurry. Specifically, in a heated state, the impurity-removed ferrous solution enters the 600-oxidation reactor and mixes with ammonia water to generate ferrous hydroxide, and then air is introduced into the 600-oxidation reactor to at least partially oxidize the ferrous hydroxide into ferric hydroxide, which is then dehydrated into ferroferric oxide to obtain iron oxide slurry, which enters the magnetic sifter.

[0107] Furthermore, the 600 oxidation reactor includes a magnetic screening device, which is used to absorb magnetic iron oxides in the iron oxide slurry and feed the magnetic iron oxides into the 700 iron phosphate synthesis reactor.

[0108] In addition, the 600-oxidation reactor has a solid-liquid separation function in addition to containing reactants for reaction. Specifically, after the magnetic screening device separates the magnetic iron oxides in the iron oxide slurry, the 600-oxidation reactor performs solid-liquid separation on the remaining slurry, and the obtained liquid product is the ammonium salt solution, which can be reserved for the production of ammonia water. The magnetic iron oxides separated by the magnetic screening device enter the 700-iron phosphate synthesis reactor.

[0109] The 700-iron phosphate synthesis reactor is used to accommodate the reaction of magnetic iron oxide and phosphoric acid under aeration conditions to generate iron phosphate. It is understandable that the 700-iron phosphate synthesis reactor, in addition to being used to accommodate the reactants for reaction, 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 generate iron phosphate, the reaction system is subjected to solid-liquid separation, and the obtained solid phase material is iron phosphate, which enters the 800-calcination reactor for calcination treatment.

[0110] The ferric phosphate preparation system provided by the present invention realizes the preparation of high-purity ferric phosphate using less equipment and has the characteristics of simple structure and low operating cost.

[0111] Figure 2 A schematic diagram of a second system for preparing iron phosphate provided by the present invention is shown in FIG. Figure 2 As shown, the system for preparing iron phosphate provided by the present invention also includes a 900 ammonia production reactor, the inlet of the 900 ammonia production reactor is 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 outlet of the 900 ammonia production reactor is respectively connected to the inlet of the 200 alkali leaching reactor and the inlet of the 600 oxidation reactor.

[0112] 900-Ammonia reactor is used to produce ammonia water, wherein the ammonia water can be produced by any of the following two methods:

[0113] The first method is that 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, and the obtained liquid phase product is an 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 produce ammonia water. Specifically, the ammonium salt and calcium hydroxide slurry react to generate ammonia gas, and the generated ammonia gas is dissolved in water to produce ammonia water.

[0114] The second method is to use ammonia water to alkali-leach the pre-oxidation tailings in a 200-alkali leaching reactor to obtain a carbon-iron mixture and a phosphorus-containing solution, and then allow the phosphorus-containing solution whose main component is diammonium dihydrogen phosphate to enter a 900-ammonia production reactor, add calcium hydroxide slurry, and produce ammonia water. Specifically, the phosphorus-containing solution and the calcium hydroxide slurry react to generate ammonia gas, and the generated ammonia gas is dissolved in water to produce ammonia water.

[0115] The setting of the 900-ammonia production reactor realizes the circulation and recovery of ammonia, and uses industrial waste as raw materials, making the low operating cost feature 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 into a 500-de-impurity reactor, adjust the pH value of the primary ferrous solution to 5.5 with ammonia water, then add polyacrylamide to the primary ferrous solution to precipitate aluminum hydroxide gel; then, add citric acid to the primary ferrous solution, the amount of citric acid added is 1.2 times the sum of the amounts of calcium and magnesium elements in the primary ferrous solution; add sodium fluoride to the primary ferrous solution at a concentration of 0.15%, react for 30 minutes under stirring, so that heavy metal impurities such as copper and zinc form insoluble precipitates, then remove the impurities generated in the above steps by filtering to obtain a de-impurity ferrous solution.

[0130] Oxidation treatment

[0131] The impurity-free ferrous solution is added to a 600-oxidation reactor, and ammonia water is gradually added thereto and air is introduced. The pH of the reaction system is controlled to be 8.2 by controlling the amount of ammonia water added, and the ORP of the reaction system is controlled to be 230 mV by controlling the aeration rate. The reaction is carried out for 50 minutes to partially oxidize the ferrous hydroxide to ferric hydroxide, and then dehydrate to generate ferroferric oxide to obtain an iron oxide slurry. The iron oxide slurry is subjected to magnetic screening treatment to obtain magnetic iron oxide and residual slurry.

[0132] Formation of iron phosphate

[0133] The magnetic iron oxide was added into a 700-iron phosphate synthesis reactor, and the magnetic iron oxide was mixed with an aqueous phosphoric acid solution under aeration conditions. The pH of the reaction system was controlled to be 4.5, and the temperature was controlled to be 65°C, and the temperature was kept for 30 minutes to ensure the initial nucleation of iron phosphate crystals. Subsequently, the system was heated to 75°C and kept warm for 30 minutes to promote the further growth of iron phosphate crystals, and finally the iron phosphate solid phase material was obtained by solid-liquid separation.

[0134] Calcination treatment

[0135] The obtained iron phosphate solid phase material was added to an 800-calcination reactor and calcined at 700° C. in an air atmosphere for 3 hours to improve its crystallinity and purity, and finally a battery-grade iron phosphate product was obtained.

[0136] Ammonia recovery and recycling

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

[0138] The ammonium salt solution is added into a 900-mm ammonia production reactor and mixed with calcium hydroxide slurry to produce ammonia water. The generated ammonia gas is dissolved in water to produce ammonia water as a raw material for alkali leaching and oxidation treatment.

[0139] In addition, it also includes using ammonia water to alkali-leach the pre-oxidation tailings in the 200-alkali leaching reactor to obtain a carbon-iron mixture and a phosphorus-containing solution, and then allowing the phosphorus-containing solution whose main component is diammonium phosphate to enter the 900-ammonia production reactor, adding calcium hydroxide slurry, and dissolving the generated ammonia gas in water to obtain ammonia water as a raw material for alkali leaching and oxidation treatment.

[0140] Test example

[0141] The ICP-OES method was used to test the impurity element content and purity of iron phosphate in the lithium iron phosphate tailings and iron phosphate. The results are shown in Table 1:

[0142] Table 1 Comparison of sample impurity content

[0143]

[0144] Among them, the tailings from lithium iron phosphate extraction contain 23.9wt% iron and 12.4wt% phosphorus.

[0145] As can be seen from Table 1, the preparation method of iron phosphate provided by the present invention can effectively remove impurities of manganese, copper, aluminum, calcium and magnesium in the tailings of lithium iron phosphate extraction, and at the same time, the prepared iron phosphate has a higher 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 it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments can still be modified, or some or all of the technical features can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing ferric phosphate, characterized in that: The steps include: Using an oxidant to pre-oxidize the lithium iron phosphate tailings to obtain pre-oxidized tailings; Using ammonia water to perform alkaline leaching on the pre-oxidation tailings to obtain a carbon-iron mixture and a phosphorus-containing solution; Performing acid leaching on the carbon-iron mixture to obtain a carbon mixture and an iron-containing solution; performing a reduction treatment on the iron-containing solution to obtain a primary ferrous solution; adding a heavy metal impurity remover to the primary ferrous solution to obtain an impurity-removed ferrous solution; 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 controlled to be 200-250 mV, and the treatment time of the oxidation treatment is 30-60 min to obtain an iron oxide slurry; Performing magnetic screening on the iron oxide slurry to obtain magnetic iron oxide and residual slurry; The magnetic iron oxide is reacted with phosphoric acid under aeration conditions, and the obtained solid phase material is calcined to obtain the iron phosphate; The oxidant is selected from at least one of ozone and hydrogen peroxide; The treatment temperature of the alkaline leaching treatment is 70-90°C, and the concentration of ammonia water is 5wt%-10wt%; The heavy metal impurity remover is selected from at least one of sodium fluoride, ammonium fluoride, sodium sulfide and ammonium sulfide.

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

3. The method for preparing ferric phosphate according to claim 2, characterized in that: In the process of allowing the magnetic iron oxide to react with phosphoric acid under aeration conditions, the ORP of the iron phosphate mixed system is controlled to be 300-350 mV.

4. The method for preparing ferric phosphate according to claim 1, characterized in that: Before adding the heavy metal impurity remover, the method further includes adding a metal ion complexing agent 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 amount of the metal ion complexing agent added is 1.1 to 1.2 times the sum of the amounts of calcium and magnesium in the primary ferrous solution; and / or, The method also includes adjusting the pH of the primary ferrous solution to 5-5.5 and adding polyacrylamide to the primary ferrous solution.

5. The method for preparing ferric phosphate according to claim 1, characterized in that: The process of reacting the magnetic iron oxide with phosphoric acid under aeration conditions further includes adding a dispersant to the iron phosphate mixed system; The dispersant is selected from at least one of polyethylene glycol, polycarboxylic acid water reducer and polyacrylamide; The dosage of the dispersant is 0.05-0.1% of the mass of the magnetic iron oxide.

6. The method for preparing ferric phosphate according to claim 1, characterized in that: The calcination treatment has a treatment temperature of 550-700° C. and a treatment time of 2-4 hours.

7. The method for preparing ferric phosphate according to claim 1, characterized in that: The method further comprises performing solid-liquid separation on the remaining slurry to obtain an ammonium salt solution; Mixing the ammonium salt solution with calcium hydroxide slurry to produce ammonia water; The ammonia water is used as a raw material for the alkaline leaching treatment and the oxidation treatment.

8. The method for preparing ferric phosphate according to claim 7, characterized in that: The method also includes mixing the phosphorus-containing solution with the calcium hydroxide slurry to produce ammonia water.

9. A system for preparing iron phosphate for executing the method for preparing iron phosphate according to any one of claims 1 to 8, characterized in that: It includes a pre-oxidation reactor, an alkali leaching reactor, an acid leaching reactor, a reduction reactor, an impurity removal reactor, an oxidation reactor, an iron phosphate synthesis reactor, and a calcination reactor which are connected in sequence; The oxidation reactor includes a magnetic screening device.

10. The system for preparing ferric phosphate according to claim 9, characterized in that: It also includes an ammonia production reactor, the inlet of which is connected to the liquid phase outlet of the alkali leaching reactor and the liquid phase outlet of the oxidation reactor respectively, and the outlet of the ammonia production reactor is connected to the inlet of the alkali leaching reactor and the inlet of the oxidation reactor respectively.

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