A process for the recovery of battery grade ferric phosphate from sludge

Abstract

This invention relates to the field of sludge resource utilization technology, and more particularly to a method for recovering battery-grade iron phosphate from sludge. The method involves incinerating dried sludge to obtain sludge incineration ash, then leaching the ash with a target acid solution of a first preset concentration. After solid-liquid separation, a phosphorus-containing leachate is obtained. An Fe source is then added to the phosphorus-containing leachate to obtain a mixed solution. This mixed solution is then subjected to hydrothermal synthesis to obtain battery-grade iron phosphate. Therefore, the above technical solution involves direct hydrothermal synthesis of the phosphorus-containing leachate with added Fe source, thus shortening the process flow for preparing battery-grade iron phosphate.

Description

Technical Field

[0001] This invention relates to the field of sludge resource utilization technology, and in particular to a method for recovering battery-grade iron phosphate from sludge. Background Technology

[0002] Pollutants and nutrients in wastewater aggregate under the action of rapidly multiplying bacteria and chemical agents, eventually forming sludge. With the rapid development of industries such as new energy vehicles and energy storage batteries, the production of battery-grade iron phosphate is also constantly increasing, leading to a greater demand for its production raw materials (such as phosphorus).

[0003] In related technologies, the main method for recovering phosphorus from sludge and preparing battery-grade iron phosphate is the wet chemical method; however, this method has a long process flow.

[0004] Therefore, there is an urgent need for a method to recover battery-grade iron phosphate from sludge to solve this technical problem. Summary of the Invention

[0005] This invention provides a method for recovering battery-grade iron phosphate from sludge, which can shorten the process flow for preparing battery-grade iron phosphate.

[0006] This invention provides a method for recovering battery-grade iron phosphate from sludge, comprising:

[0007] The dried sludge is incinerated to obtain sludge incineration ash;

[0008] The sludge incineration ash is leached using a target acid solution of a first preset concentration, and a phosphorus-containing leachate is obtained after solid-liquid separation.

[0009] A Fe source was added to the phosphorus-containing leachate to obtain a mixed solution;

[0010] The mixed solution was subjected to hydrothermal synthesis to obtain battery-grade iron phosphate.

[0011] As can be seen from the above scheme, the method for recovering battery-grade iron phosphate from sludge provided by the present invention involves incinerating dried sludge to obtain sludge incineration ash, then leaching the sludge incineration ash with a target acid solution of a first preset concentration, and obtaining a phosphorus-containing leachate after solid-liquid separation. A Fe source is then added to the phosphorus-containing leachate to obtain a mixed solution, which is then subjected to hydrothermal synthesis to obtain battery-grade iron phosphate. Therefore, the above technical solution involves direct hydrothermal synthesis treatment of the phosphorus-containing leachate with added Fe source, thus shortening the process flow for preparing battery-grade iron phosphate. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a schematic diagram of a method for recovering battery-grade iron phosphate from sludge according to an embodiment of the present invention;

[0014] Figure 2 A comparison chart of the recovered ferric phosphate and commercial-grade ferric phosphate provided in this embodiment of the invention;

[0015] Figure 3 This is a diagram illustrating the thermodynamic equilibrium calculation system for the precipitation reaction between impurity components and target phosphorus-containing products in a phosphorus-containing leachate, provided in an embodiment of the present invention.

[0016] Figure 4 The Gibbs free energy variation diagram of the ferric phosphate precipitation reaction provided in the embodiments of the present invention;

[0017] Figure 5 The reaction equilibrium constant diagram for the ferric phosphate precipitation reaction provided in the embodiments of the present invention is shown. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1 One embodiment of the present invention provides a method for recovering battery-grade iron phosphate from sludge, the method comprising:

[0020] Step 100: Incinerate the dried sludge to obtain sludge incineration ash;

[0021] Step 102: Use the target acid solution of the first preset concentration to leach the sludge incineration ash, and obtain the phosphorus-containing leachate after solid-liquid separation;

[0022] Step 104: Add Fe source to phosphorus-containing leachate to obtain mixed solution;

[0023] Step 106: Perform hydrothermal synthesis on the mixed solution to obtain battery-grade iron phosphate.

[0024] In this embodiment, dried sludge is incinerated to obtain sludge incineration ash. The sludge incineration ash is then leached with a target acid solution of a first preset concentration. After solid-liquid separation, a phosphorus-containing leachate is obtained. An Fe source is then added to the phosphorus-containing leachate to obtain a mixed solution. This mixed solution is then subjected to hydrothermal synthesis to obtain battery-grade iron phosphate. Therefore, the above technical solution involves direct hydrothermal synthesis of the phosphorus-containing leachate with added Fe source, thus shortening the process flow for preparing battery-grade iron phosphate.

[0025] In some implementations, the incineration temperature is 750–900°C to ensure the incineration effect of the dried sludge.

[0026] In one embodiment of the present invention, the target acid solution includes at least one of sulfuric acid and nitric acid.

[0027] In this embodiment, the target acid solution can better leach phosphorus from sludge incineration ash, and at the same time, it can suppress the co-precipitation of metal impurities that are leached simultaneously during the hydrothermal synthesis process, ensuring that only phosphorus and iron elements precipitate.

[0028] In related technologies, during phosphorus leaching, elements such as Fe, Ca, Mg, and Al in the sludge leach out simultaneously with phosphorus. To avoid these elements forming impurities, they are often removed through methods such as pH pre-precipitation or cation exchange resin methods. The pH is then adjusted to the optimal precipitation range for the target product to obtain the highest possible yield. This method often requires 6-8 detailed steps to purify the leachate and adjust the pH, greatly increasing the complexity of the entire phosphorus recovery process. However, the target acid solution with the first preset concentration in this embodiment can recover phosphorus under conditions of low extract concentration, no need for pH adjustment, and no filtrate purification, effectively simplifying the processing flow and reducing processing costs.

[0029] Thermodynamic calculations show that ferric phosphate exhibits an independent precipitation range under relatively strong target acid concentration conditions with a pH of 0.4–2.1. Figure 3 (The width of the horizontal bar for each substance represents the degree of precipitation reaction under the corresponding pH conditions.) This means that other phosphorus-containing products (such as AlPO4, MgNH4PO4, Ca5(PO4)3OH) and solid impurities (such as Fe(OH)3, Al(OH)3, Mg(OH)2, Ca(OH)2) cannot undergo precipitation reactions within this range. Therefore, by controlling the first preset concentration of the target acid solution, i.e., at least one of sulfuric acid and nitric acid, the pH of the phosphorus-containing leachate can be guaranteed to achieve separate precipitation of ferric phosphate within this range, thereby ensuring the purity of ferric phosphate.

[0030] In one embodiment of the present invention, the first preset concentration is 0.02 to 0.15 mol / L, which can ensure the leaching effect of the target acid solution with the first preset concentration on phosphorus.

[0031] In some embodiments, the solid-liquid ratio of sludge incineration ash to target acid solution of a first preset concentration can be 1:50 to 1:200, the leaching time can be 2 to 4 hours, and the oscillation rate can be 60 to 120 rpm, so as to ensure the leaching effect of the target acid solution of the first preset concentration on phosphorus in sludge incineration ash.

[0032] In some embodiments, the phosphorus-containing leaching solution can be filtered using a filter head such as 0.45 μm to remove any solid particles that have not been properly separated.

[0033] In one embodiment of the present invention, a phosphorus-containing leachate is used for the cyclic leaching of new sludge incineration ash.

[0034] In this embodiment, the phosphorus in the phosphorus-containing leachate is recycled to the sludge incineration ash, which reduces reagent consumption.

[0035] In one embodiment of the present invention, the Fe source includes at least one of FeCl3, Fe(NO3)3 and Fe2(SO4)3.

[0036] In this embodiment, the Fe source is an inorganic compound, which can make the mixed solution have a strong tendency to crystallize, which is conducive to the formation of crystals.

[0037] In one embodiment of the present invention, the molar ratio of Fe source and phosphorus element in phosphorus-containing leachate is 1 to 1.05:1.

[0038] In one embodiment of the present invention, the hydrothermal synthesis reaction temperature is 150–210°C and the reaction time is 1–24 h.

[0039] In one embodiment of the present invention, step 106 may specifically include:

[0040] The mixed solution was subjected to hydrothermal synthesis to obtain the hydrothermal synthesis product;

[0041] The hydrothermal synthesis product was subjected to solid-liquid separation to obtain a solid product;

[0042] The solid product is washed with a washing solution, and then the washed solid product is freeze-dried.

[0043] The freeze-dried solid product was calcined in air to obtain battery-grade iron phosphate.

[0044] In this embodiment, under the heating condition of the target acid solution, the precipitation reactions of relevant impurities and phosphorus-containing products other than ferric phosphate can be further suppressed. It should be noted that the precipitation reaction of ferric phosphate is a reversible reaction at low temperatures. In reversible reactions, the equilibrium constant lgK < 5, which leads to a decrease in the yield of ferric phosphate and a slower synthesis rate. However, the hydrothermal method can effectively increase the reaction equilibrium constant lgK at high temperatures, making the synthesis reaction more complete and faster. Figure 4 (When △G < 0, the reaction is considered to proceed spontaneously) and Figure 5 (When lgK>5, the precipitation reaction is considered to be more complete).

[0045] Furthermore, the hydrothermal-generated iron phosphate exhibits a uniform micron-sized spherical structure, which demonstrates superior electrochemical performance compared to the amorphous structure obtained through room-temperature precipitation. Simultaneously, iron phosphate does not grow directly through nucleation in a liquid environment but rather adheres tightly to the wall surface. This characteristic allows for excellent thin-layer deposition during the reaction process, further contributing to its enhanced electrochemical performance.

[0046] In this embodiment, the iron phosphate particles are all microscopically regular spheres with a sphericity > 0.9 and an average particle size < 1 μm. The Fe, P, and O elements exhibit a uniform symbiotic distribution (e.g., ...). Figure 2 This is to give iron phosphate better electrochemical performance and ion conduction.

[0047] exist Figure 2 In the diagram, a and b represent a comparison of the macroscopic morphology of ferric phosphate produced by this method and commercial-grade ferric phosphate, respectively; c and d represent a comparison of the microscopic morphology of ferric phosphate produced by this method and commercial-grade ferric phosphate, respectively; and e and f represent a comparison of the crystal structure of ferric phosphate produced by this method and commercial-grade ferric phosphate, respectively. The comparison is achieved through... Figure 2 It can be seen that the iron phosphate prepared in the embodiments of the present invention has greatly improved uniformity.

[0048] In this embodiment of the invention, the iron phosphate can meet the standard of HG / T4701-2021 for iron phosphate used in batteries, and the data is shown in Table 1 (showing the content of battery-grade iron phosphate, analytical reagent grade and the product recovered by this method).

[0049] Table 1 Comparison of Ferric Phosphate and Standard Limits

[0050]

[0051] In this embodiment, vacuum filtration is used to separate the hydrothermal synthesis products into solid and liquid components, so as to achieve better separation of the solid and liquid components of the hydrothermal synthesis products.

[0052] In some embodiments, the calcination temperature can be 500-800°C to allow the freeze-dried solid product to be fully calcined to remove the water of crystallization, thereby obtaining battery-grade iron phosphate.

[0053] In one embodiment of the present invention, the washing liquid includes a first washing liquid and a second washing liquid, wherein the first washing liquid is dilute nitric acid or dilute sulfuric acid of a second preset concentration, and the second washing liquid is deionized water.

[0054] In this embodiment, the washing step in the hydrothermal process is mainly to remove impurity ions carried by residual moisture in the solid product. Experiments show that when the Al content in the sludge is too high (e.g., Al salt conditioner was added during the front-end treatment of the sludge), Al doping may still occur during the hydrothermal synthesis process. In this case, washing with dilute nitric acid or dilute phosphoric acid can remove the impurity without causing loss of ferric phosphate. Here, the PO4 in ferric phosphate is utilized. 3- Compared to NO3 - It has strong coordination ability and is chemically insoluble in dilute nitric acid.

[0055] In some embodiments, the second preset concentration is 1%, which can ensure the washing effect of the second preset concentration of dilute nitric acid or dilute sulfuric acid on the solid product.

[0056] In one embodiment of the present invention, washing a solid product with a washing liquid includes: washing the solid product sequentially with a first washing liquid and a second washing liquid.

[0057] In some embodiments, if the sludge is Al salt conditioning sludge, the solid product can be washed twice with 1% dilute nitric acid or 1% dilute phosphoric acid during the hydrothermal synthesis process, and then washed with deionized water. It should be noted that the washing liquid can be reused, which can reduce the cost of the reagents.

[0058] In one embodiment of the present invention, after freeze-drying the washed solid product, the method further includes:

[0059] Add a reducing agent and a Li source to the freeze-dried solid product;

[0060] The solid product containing a reducing agent and a Li source was heated under an inert atmosphere to obtain battery-grade lithium iron phosphate.

[0061] In this embodiment, the recovered product is battery-grade iron phosphate with high sphericity. After further thermochemical addition of Li, high-purity lithium iron phosphate can be obtained, realizing the resource utilization from sludge to high-value phosphorus-containing products.

[0062] In some embodiments, battery-grade lithium iron phosphate can be obtained by heating the solid product containing a reducing agent and a Li source at 600–800°C for 10 hours in an inert atmosphere.

[0063] In one embodiment of the present invention, the reducing agent includes at least one of H2 and a carbon source, wherein the carbon source includes at least one of glucose, sucrose, starch and carbon black, and the Li source includes at least one of LiOH and Li2CO3.

[0064] In one embodiment of the present invention, when the reducing agent is H2, the gas volume ratio of the reducing agent to the inert gas in the solid product is 5%:95%.

[0065] In one embodiment of the present invention, when the reducing agent is a carbon source, the amount of reducing agent added is 0.5 to 10% of the total mass of the freeze-dried solid product.

[0066] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recovering battery-grade iron phosphate from sludge, characterized in that, include: The dried sludge is incinerated to obtain sludge incineration ash; The sludge incineration ash is leached using a target acid solution of a first preset concentration, and a phosphorus-containing leachate is obtained after solid-liquid separation. A Fe source was added to the phosphorus-containing leachate to obtain a mixed solution; The mixed solution was subjected to hydrothermal synthesis to obtain battery-grade iron phosphate; The target acid solution includes at least one of sulfuric acid and nitric acid, and the first preset concentration is 0.02~0.15 mol / L to ensure that the pH of the phosphorus-containing leachate is 0.4~2.1; The hydrothermal synthesis reaction temperature is 150~210℃, and the reaction time is 1~24h; The hydrothermal synthesis of the mixed solution to obtain battery-grade iron phosphate includes: The mixed solution was subjected to hydrothermal synthesis to obtain the hydrothermal synthesis product; The hydrothermal synthesis product was subjected to solid-liquid separation to obtain a solid product; The solid product is washed with a washing solution, and the washed solid product is freeze-dried. The freeze-dried solid product was calcined in air to obtain battery-grade iron phosphate. The washing solution includes a first washing solution and a second washing solution, wherein the first washing solution is dilute nitric acid or dilute sulfuric acid of a second preset concentration, and the second washing solution is deionized water; The washing of the solid product with a washing solution includes: The solid product is washed sequentially using the first washing solution and the second washing solution.

2. The method according to claim 1, characterized in that, The phosphorus-containing leachate is used for the cyclic leaching of new sludge incineration ash.

3. The method according to claim 1, characterized in that, The Fe source includes at least one of FeCl3, Fe(NO3)3 and Fe2(SO4)3, and the molar ratio of the Fe source to phosphorus in the phosphorus-containing leachate is 1~1.05:

1.

4. The method according to claim 1, characterized in that, After freeze-drying the washed solid product, the process further includes: Add a reducing agent and a Li source to the freeze-dried solid product; The solid product containing a reducing agent and a Li source was heated under an inert atmosphere to obtain battery-grade lithium iron phosphate.

5. The method according to claim 4, characterized in that, The reducing agent includes at least one of H2 and a carbon source, wherein the carbon source includes at least one of glucose, sucrose, and starch, and the Li source includes at least one of LiOH and Li2CO3.

6. The method according to claim 5, characterized in that, When the reducing agent is H2, the volume ratio of the reducing agent to the inert gas in the solid product is 5%:95%; and / or, When the reducing agent is a carbon source, the amount of reducing agent added is 0.5-10% of the total mass of the freeze-dried solid product; and / or, The molar ratio of Fe source to Li source in the solid product is 1:1.05.

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