Process for preparing sodium iron phosphate pyrophosphate positive electrode material from phosphorus iron slag

Abstract

This invention belongs to the field of secondary resource recycling and sodium-ion batteries, and discloses a process for recovering phosphorus and iron elements from iron phosphate slag to produce sodium iron phosphate pyrophosphate cathode material. The iron phosphate slag is a secondary resource containing phosphorus and iron elements, mainly composed of Fe and P, with additional elements such as Cu, Al, Li, Si, and Ca. First, the iron phosphate slag is subjected to molten salt chlorination treatment to selectively extract iron elements. Second, taking advantage of the fact that phosphorus and sodium elements in the chlorination final product are soluble, water leaching separation is performed to extract phosphorus and sodium elements. Finally, a precursor is prepared using a controlled mixture of iron, phosphorus, and sodium elements. The precursor material is then calcined under an inert atmosphere to produce sodium iron phosphate pyrophosphate cathode material.

Description

Technical Field

[0001] This invention belongs to the field of secondary resource recycling and sodium-ion batteries, and more specifically, relates to a process for preparing sodium iron phosphate pyrophosphate cathode material by recycling iron phosphate slag. Background Technology

[0002] As the concept of low carbon matures, building a low-carbon energy system is an inevitable step towards achieving the goals of "carbon neutrality and carbon peaking." Compared to lithium-ion batteries, which are subject to resource constraints, sodium-ion batteries have widely available raw materials, superior performance, and broad application prospects in fields such as power batteries and large-scale energy storage, making them a powerful tool for achieving the "carbon neutrality and carbon peaking" goals. Similarly, resource recycling is becoming more integrated into the low-carbon energy system, and improving the high-level recycling of secondary resources is an inevitable trend.

[0003] Currently, the process for recycling resources to prepare battery cathode materials mainly involves recovering valuable elements from waste lithium-ion battery black powder and then synthesizing them into cathode materials. This method not only suffers from high equipment requirements, long recycling processes, and difficulties in element separation, but also has high requirements for raw materials and is not widely applicable to other phosphorus- and iron-containing secondary resources. For example:

[0004] Chinese patent CN115818613B discloses a method for preparing carbon-coated sodium fluorophosphate from waste lithium iron phosphate. This method utilizes chlorination roasting to treat waste lithium-ion batteries, requires the use of vacuum equipment and connection to an induced draft fan, places high demands on the equipment, and is not suitable for secondary resources such as phosphorus-containing iron smelting slag with complex composition.

[0005] In view of this, a method for recycling phosphorus and iron secondary resources that can process most phosphorus and iron slag (containing phosphorus and iron elements as secondary resources) with a short process and high added value of the final product is proposed. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention provides a method for preparing sodium iron pyrophosphate and its application. Using ferrophosphate slag (a secondary resource containing ferrophosphate elements) as raw material as iron and phosphorus sources, the method firstly utilizes molten salt chlorination to selectively recover the iron source by high-temperature chlorination of the ferrophosphate slag. Secondly, the roasted slag is water-leached, and the leaching conditions are controlled to obtain phosphorus and sodium sources. Finally, based on the recovered phosphorus, sodium, and iron sources, a sodium iron pyrophosphate precursor is prepared, and the precursor is heat-treated to obtain the product.

[0007] To achieve the objectives of this invention, the specific technical solution is as follows:

[0008] The method for preparing sodium iron pyrophosphate cathode material for sodium-ion batteries provided by this invention includes the following steps:

[0009] Step 1, Treatment of iron-phosphorus slag: The iron-phosphorus slag is crushed, ground, and then leached with sodium hydroxide solution to remove aluminum. After filtration, the filter residue is washed, dried, and sodium salt and chlorinating agent are added. The mixture is then ground until the particle size is less than 300 mesh to obtain material 1.

[0010] Step 2, Iron source treatment: Material 1 is subjected to molten salt chlorination, and the volatiles are collected to obtain iron chlorination product 2; the molten salt chlorination residue is chlorination residue 3;

[0011] Step 3, Preparation of iron source solution: Gas ferric chloride 2 is passed into deionized water and stirred to dissolve, and excess iron powder is added;

[0012] Step 4, Treatment of sodium and phosphorus sources: The chlorinated residue 3 is leached with water. During the stirring leaching, excess iron powder is added and CO2 is introduced. The mixture is filtered while hot, and the filtrate is evaporated, concentrated, cooled, and crystallized to obtain phosphorus and sodium sources.

[0013] Step 5, Precursor preparation: Mix the iron source solution, phosphorus source and sodium source, and add additional sodium source and phosphorus source to adjust the ratio of iron, sodium and phosphorus elements in the mixture. Add a certain amount of reducing agent and carbon source, control the temperature and stirring speed, and spray dry to obtain the precursor.

[0014] Step 6, Preparation of sodium iron pyrophosphate material: The dried precursor material is sintered in an inert atmosphere, cooled in the furnace, removed and ground to obtain the desired sodium iron pyrophosphate cathode material.

[0015] Furthermore, the phosphorus-iron slag mentioned in step 1 can be secondary resources containing phosphorus and iron elements, such as steelmaking and ironmaking slag, phosphorus chemical by-products, and waste lithium iron phosphate extraction slag; the alkaline solution concentration is 20-30% sodium hydroxide solution, the stirring speed is controlled at 40-200 r / min during the alkaline leaching process, and the stirring time is 30-90 min; the filter residue is washed 2-4 times, and the washing liquid is returned to the alkaline leaching solution; the filter residue drying temperature is 100-120℃; the essential sodium salt added after the filter residue is dried is sodium chloride; other sodium salts are one or more of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium citrate, and sodium ascorbate; the chlorinating agent is one or more of ammonium chloride, carbon tetrachloride, and chlorine; the ratio of sodium salt added to material mass is 1:1.5-2; the proportion of sodium chloride in the sodium salt is 0.5-0.7; the ratio of chlorinating agent added to material mass is 1:1.5-3;

[0016] Furthermore, in step 2, the molten salt chlorination temperature is 500-900℃, the iron chloride compound is mainly composed of FeCl3 and contains a small amount of NaFeCl4 complex chloride, and a cooling pipe is installed outside the calcined quartz tube to gradually cool from 750℃ to a final cooling temperature of 240-280℃.

[0017] Furthermore, in step 3, the stirring speed is 200-300 r / min, and the added iron powder is in excess, being 1.05-1.1 times the molar amount of iron in the raw material;

[0018] Furthermore, in step 4, the water immersion stirring speed is 200–400 r / min, and the gas introduction rate is 0.1–0.3 m / min. 3 The water immersion temperature was 80-120℃, and the hot filtration method was vacuum filtration. The evaporation, concentration, cooling and crystallization yielded a sodium phosphate compound mainly composed of sodium dihydrogen phosphate hydrate.

[0019] Furthermore, in step 5, the molar ratio of sodium, iron, and phosphorus is 3.9–4.1:2.9–3.1:3.9–4.1; the additional sodium source is one or more of sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, and sodium acetate; the additional phosphorus source is phosphoric acid; the reducing agent is one or more of oxalic acid, citric acid, ascorbic acid, and glucose; the carbon source is one or more of glucose, starch, graphene, acetylene black, and gelatin; the solution temperature is controlled at 80–90°C, and the stirring speed is 200–300 r / min.

[0020] Furthermore, in step 6, the inert atmosphere is argon or nitrogen. During sintering, the sintering is first performed at a temperature of 300–350°C for 1–5 hours, and then sintered at a temperature of 450–600°C for 5–20 hours, with a heating rate of 1–10°C / min.

[0021] The main technical principles of this invention are as follows:

[0022] Principle of chlorination process:

[0023] Under atmospheric conditions, sodium chloride is used as the molten salt, supplemented by a partial chlorinating agent, for molten salt chlorination. The reaction principle is as follows:

[0024] Molten NaCl acts as a molten salt, providing chloride ion free radicals. Simultaneously, the added chlorinating agent decomposes to produce chlorine gas. Chlorine gas or chloride ions react chemically with Fe oxides to generate FeCl3, which volatilizes (FeCl3 is gaseous at high temperatures). This process also regulates the presence of chloride ion complexes for synergistic extraction.

[0025] Complexation reactions that occur in a chlorination system using NaCl as the molten salt:

[0026] NaCl + FeCl3 = NaFeCl4

[0027] The vapor pressure of the composite chloride NaFeCl4 is higher than that of the pure chloride FeCl3, and it has a lower boiling point than the pure chloride, which can achieve separation and recovery from impurities. At the same time, by taking advantage of the large difference in boiling points of each chloride, the preferential chlorination extraction and separation of iron can be achieved.

[0028] The reaction principle during water immersion:

[0029] Sodium and phosphorus elements in the roasting residue enter the solution in a soluble form, while carbon dioxide is introduced to maintain a weakly acidic environment; iron powder is added to remove copper elements dissolved by chlorination, and the iron in the solution is oxidized and reacts with phosphate ions to precipitate and remove the copper.

[0030]

[0031] The solution is filtered while hot, and the filtrate is evaporated, concentrated, cooled, and crystallized to obtain sodium dihydrogen phosphate; while impurities remain in the residue.

[0032] Compared with existing technologies, the advantages of this invention are:

[0033] (1) This invention uses molten salt chlorination to treat secondary resources containing phosphorus and iron. During the chlorination process, iron is collected in the form of chloride, which achieves selective recovery of iron and avoids it from entering further separation steps, greatly reducing the difficulty of separation and purification. The chlorination process is an exothermic reaction, which can reduce the energy consumption of the recovery process and strengthen the low-carbon goal.

[0034] (2) This invention has low requirements for the selectivity of material composition and is universally applicable. For secondary resources containing Fe, P, Al, Li, Cu, Ca and Si elements, such as steelmaking slag, electric furnace yellow phosphorus slag and waste lithium iron phosphate extraction slag, the recycling process of "prioritizing aluminum removal - chlorination for iron extraction - water leaching for sodium phosphate extraction" can be adopted.

[0035] (3) The extracted Fe, P and Na elements can be well connected to the preparation of sodium iron pyrophosphate cathode material by liquid phase method, so as to realize the high added value utilization of phosphorus-containing iron secondary resources. Attached Figure Description

[0036] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0037] Figure 1 This is a schematic diagram of the process flow of the present invention;

[0038] Figure 2 This is a SEM image of the cathode material prepared according to the present invention;

[0039] Figure 3 This is an XRD diagram of the cathode material prepared according to the present invention. Detailed Implementation

[0040] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0041] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0042] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0043] Example 1

[0044] The content of phosphorus-containing iron secondary resource elements used is as follows:

[0045] element Fe Al Cu Ca Si C P content(%) 39.75 4.74 5.32 4.39 3.76 19.34 22.7

[0046] A method for preparing sodium ferric pyrophosphate from ferric phosphorus by secondary resource recovery based on molten salt chlorination is shown in the process flow diagram below. Figure 1 As shown, it includes the following steps:

[0047] Step 1: Weigh 100g of ferrophosphate slag (a secondary resource containing ferrophosphate from steelmaking slag in a certain factory). Crush the ferrophosphate slag and grind it to a particle size of less than 300 mesh. Then add it to a stirrer with a rotation speed of 60 r / min and remove aluminum by alkaline leaching with 25% sodium hydroxide solution. The solid-liquid ratio is controlled at 6:1. After stirring for 45 min, filter the residue and wash it twice. Return the filtrate to the alkaline leaching solution. Dry the residue at 120℃ and add 65g of sodium salt, including 50g of sodium chloride, 10g of sodium carbonate, and 5g of sodium hydroxide; add 35g of solid chlorinating agent, including 25g of ammonium chloride and 10g of sodium hypochlorite. Mix well and grind to a particle size of less than 300 mesh to obtain material 1.

[0048] Step 2: Place material 1 in a tube furnace. During the molten salt chlorination process, control the calcination temperature at 750℃, the calcination holding time at 1 hour, and the heating rate at 5℃ / min. Air is introduced into the tube furnace at a constant rate of 75ml / min to blow out the volatiles.

[0049] Step 3: The gaseous iron chloride from Step 2 is passed through a gradually cooling pipe, with the final temperature controlled at 240-280℃, into deionized water to dissolve. Excess iron powder is added, and the stirring speed is controlled at 210 r / min to obtain a recovered iron-sodium solution. After calcination, the chloride residue is removed and sent to the leaching process.

[0050] Step 4: Take all the roasted residue, add deionized water, control the solid-liquid ratio at 1:3 to 4, stir at 270 r / min, and leach at 85 to 90℃.

[0051] Carbon dioxide is passed through the filtrate at a rate of 0.2–0.22 m / s. 3 The stirring rate is required to be 30 minutes under leaching conditions, followed by filtration to remove calcium and silicon slag.

[0052] Add excess iron powder to the filtrate, keep other conditions unchanged, stir for 30 minutes under leaching conditions, and filter to remove copper and excess iron powder;

[0053] The filtrate was stirred continuously, and the carbon dioxide was stopped and replaced with air at a rate of 0.25–0.27 m / s. 3 The leaching process involves stirring for 30 minutes under leaching conditions, followed by filtration under vacuum while the solution is still hot. The ferrous ions are oxidized and precipitated as ferric phosphate for recovery. The filtrate is then evaporated and concentrated at 120–125°C, and cooled to 60–70°C for crystallization and purification to obtain sodium phosphate hydrate as the main component.

[0054] Step 5: Mix the iron source solution and the sodium phosphorus source in a certain proportion, and add additional sodium source sodium ascorbate and phosphoric acid source phosphate to ensure that the molar ratio of sodium, iron and phosphorus in the solution is 3.9-4.1:2.9-3.1:3.9-4; add 10% of the iron molar amount of ascorbic acid as a reducing agent, and add 7% of the iron molar amount of glucose as a carbon source; stir at 230 r / min for 1 hour at 80°C to fully mix, and obtain a slurry; spray dry the slurry to obtain the precursor material; the spray drying inlet temperature is 160°C and the outlet temperature is 80°C.

[0055] Step 6: Under an argon atmosphere, the precursor material is pre-sintered at 300°C for 1 hour at a heating rate of 3°C / min, then heated to 500°C for sintering for 10 hours, and cooled in the furnace to obtain sodium iron pyrophosphate cathode material.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A process for recovering ferrophosphate slag to prepare sodium iron phosphate pyrophosphate cathode material, characterized in that: Includes the following steps: 1) The phosphorus-iron slag is crushed, ground, and then leached with sodium hydroxide solution to remove aluminum. After filtration, the filter residue is washed, dried, and then sodium salt and chlorinating agent are added. The mixture is then ground until the particle size is less than 300 mesh to obtain material 1; 2) Material 1 is subjected to molten salt chlorination, and the volatiles are collected to obtain gaseous iron chloride product; the molten salt chlorination residue is the chlorination residue; 3) The gaseous iron chloride is passed into deionized water and stirred to dissolve, and excess iron powder is added to obtain an iron source; 4) The chlorination residue is leached with water, and excess iron powder is added during stirring and CO2 is introduced. After filtration, filter residue #1 is obtained. 5) Air is introduced into the filtrate to oxidize and precipitate iron. The mixture is then filtered while hot to obtain filter residue #2 and filtrate #2. The filtrate #2 is evaporated, concentrated, cooled, and crystallized to obtain sodium dihydrogen phosphate. 6) The iron source and sodium dihydrogen phosphate are mixed, and additional sodium and phosphorus sources are added according to the solution composition to adjust the ratio of iron, sodium, and phosphorus elements. A certain amount of reducing agent and carbon source are added, and the temperature and stirring speed are controlled. The mixture is then spray-dried to obtain the precursor. 7) The dried precursor material is sintered in an inert atmosphere, cooled in the furnace, removed, and ground to obtain sodium iron phosphate pyrophosphate cathode material.

2. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 1), the phosphorus slag is one or more of steelmaking and ironmaking slag, phosphorus chemical by-products, and waste lithium iron phosphate extraction slag; the sodium hydroxide solution concentration is 20-30%, the stirring speed is controlled at 40-200 r / min during the alkali leaching process, and the stirring time is 30-90 min; the filter residue is washed 2-4 times, and the washing liquid is returned to the alkali leaching; the filter residue drying temperature is 100-120℃; the sodium salt is one or more of sodium carbonate, sodium bicarbonate, sodium citrate, and sodium ascorbate, as well as sodium chloride; the chlorinating agent is one or more of ammonium chloride, carbon tetrachloride, and chlorine; the ratio of sodium salt added to material mass is 1:1.5-2; the mass ratio of sodium chloride to sodium salt in the sodium salt is 0.5-0.7:1; the ratio of chlorinating agent added to material mass is 1:1.5-3.

3. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 2), the molten salt chlorination temperature is 500–900°C, and the gaseous iron chloride obtained by chlorination is cooled from 750°C through a gradual cooling pipe, with the final cooling temperature being 240–280°C; the cooling pipe is a calcined quartz tube.

4. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 3), the stirring speed is 200-300 r / min, and the added iron powder is in excess, which is 1.05-1.1 times the molar amount of iron in the raw material.

5. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 4), the water immersion stirring speed is 200–400 r / min, and the CO2 introduction rate is 0.1–0.3 m. 3 The flow rate is 80-120℃, and the hot filtration method uses vacuum filtration.

6. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 5), the ratio of iron, sodium, and phosphorus is adjusted to a molar ratio of sodium, iron, and phosphorus of 3.9–4.1:2.9–3.1:3.9–4.

1. The additional sodium source is one or more of sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, and sodium acetate, and the additional phosphorus source is phosphoric acid. The reducing agent is one or more of oxalic acid, citric acid, ascorbic acid, and glucose. The carbon source is one or more of glucose, starch, graphene, acetylene black, and gelatin. The solution temperature is controlled at 80–90°C, and the stirring speed is 200–300 r / min.

7. The process for preparing sodium iron phosphate pyrophosphate cathode material by recovering ferrophosphate slag according to claim 1, characterized in that: In step 6), the inert atmosphere is argon or nitrogen. During sintering, the sintering is first pre-sintered at 300-350℃ for 1-5 hours, and then sintered at 450-600℃ for 5-20 hours. The heating rate is 1-10℃ / min.

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