A method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate

Abstract

The present invention belongs to the field of battery waste residue treatment, and specifically relates to a method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate. The lithium iron phosphate lithium-extracted slag and a binder are mixed and pelletized, and then heat-treated to obtain a pre-treated pellet. The pre-treated pellet is mixed with a carbonaceous material, additive A, and additive B, and heat-insulated under a reducing atmosphere and at a temperature T1. The slag is removed and a ferrophosphorus alloy is separated. The additive A contains silicon dioxide; the additive B contains a metal M oxide, and the M includes at least one of Ca, Mg, and Al; the weight ratio of additive B to additive A is 1.0 to 2.0; the temperature T1 is above 1550 °C. The ferrophosphorus alloy is subjected to acid treatment and calcination treatment to obtain battery-grade iron phosphate. The iron recovery rate of the slag in the present invention is high. In addition, high-quality battery-grade iron phosphate can be prepared in a resource-based manner.

Description

Technical Field

[0001] The invention belongs to the field of recycling waste battery materials, and specifically relates to the field of resource utilization of waste lithium iron phosphate lithium extraction slag. Background Art

[0002] Lithium iron phosphate batteries (LiFePO4) are widely used in mobile electronic devices, new energy vehicles, and energy storage base stations due to their excellent electrochemical properties and high safety. The life of power batteries for new energy vehicles is usually only 3 to 5 years, and there will inevitably be an explosive wave of scrapping of lithium iron phosphate batteries in the future. Due to its high value, lithium is usually preferentially extracted. At present, the lithium resources in lithium iron phosphate positive electrode materials are generally converted into lithium salt solutions by wet methods, and then recovered in the form of lithium carbonate, retaining FePO4 and other impurities in the lithium extraction slag. However, the Fe and P elements in the lithium extraction slag are very likely to cause heavy metal pollution and eutrophication of water bodies. If they are not properly disposed of, they will cause new environmental problems and greatly waste Fe and P resources.

[0003] If the impurities in lithium iron phosphate extraction slag can be effectively separated and the iron and phosphorus in the lithium iron phosphate extraction slag can be recovered in the form of battery-grade iron phosphate precursor, it will not only achieve high-value recycling of lithium iron phosphate extraction slag, but also promote the sustainable development of the lithium iron phosphate battery industry. At present, many experimental technologies have emerged for the recycling of lithium iron phosphate extraction slag, such as magnetic separation and acid leaching.

[0004] For example, a Chinese patent document with publication number CN 116588909 A discloses a method for preparing high-quality low-phosphorus iron concentrate from lithium iron phosphate slag in a short process, wherein the ferrophosphorus slag after lithium extraction from waste lithium iron phosphate is pulped, concentrated sulfuric acid is added, heated and stirred, reduction acid leaching is performed, heated and stirred, filtered, and washed to obtain an acid leaching solution and acid leaching residue, a reducing substance is added to the acid leaching solution, and then a buffer solution is added to adjust the hydrogen ions of the acid leaching solution, the heating and stirring are continued, the reaction is continued, and the ferrous phosphate solution and metal slag are obtained, the ferrous phosphate solution is acidified, heated and stirred, an oxidizing substance is added, filtered, and washed to obtain an amorphous ferric phosphate dihydrate.

[0005] The Chinese patent document with publication number CN 116425136 A discloses a method for purifying and recovering battery-grade iron phosphate from ferrophosphorus slag after lithium extraction, crushing the ferrophosphorus slag after lithium extraction, magnetic separation, adding acid solution for leaching, solid-liquid separation, supplementing phosphorus source or iron source, adding sodium nitrite and hydrogen peroxide for reaction, adjusting hydrogen ions, diluting, solid-liquid separation, and obtaining iron phosphate after high-temperature sintering. This technology can avoid the use of aluminum removal agents, fluoride salts and other substances, deeply remove metal ions, and obtain higher purity iron phosphate. However, the above two methods are complicated and do not effectively remove impurities, and the obtained iron phosphate does not meet the battery grade requirements.

[0006] The Chinese patent document with the publication number CN 114195112 A discloses an innovative recycling method for waste lithium iron phosphate batteries. This method aims to efficiently extract and produce battery-grade lithium carbonate, iron phosphate, or high-purity sodium phosphate products from waste batteries. In this process, first, the lithium element in waste lithium iron phosphate batteries is extracted using dilute acid leaching technology. Subsequently, for the lithium iron phosphate lithium extraction residue after lithium extraction, an alkali leaching method is used to further extract sodium phosphate. However, in the subsequent treatment of the iron residue, although roasting treatment is directly carried out to obtain iron concentrate powder, due to insufficient deep removal of phosphorus element, the main component of the obtained iron concentrate powder is high-phosphorus and low-grade iron oxide red, which to a certain extent limits its market added value and application scope.

[0007] The Chinese patent document with the publication number CN 115784187 A discloses a new method for preparing anhydrous iron phosphate from iron-phosphorus slag. This method uses an alkali leaching method to effectively separate iron and phosphorus elements, enabling the iron element to be enriched in the slag while the phosphorus element enters the alkali leaching solution. Subsequently, the iron slag undergoes acid dissolution treatment to generate an iron-containing acid solution; at the same time, the alkali leaching solution is purified to obtain a pure phosphorus-containing alkali solution. After mixing these two types of solutions, basic iron phosphate is successfully generated. However, this process has not achieved deep removal of phosphorus in the iron slag, and the entire preparation process is relatively cumbersome, still requiring further optimization in practical applications to improve efficiency and reduce costs.

[0008] In summary, among the resource utilization methods of waste lithium iron phosphate materials, the magnetic separation method is only applicable to slag samples with a relatively high iron content, and the recovery effect is poor. Moreover, the acid leaching process consumes a large amount of acid, the leaching effect of iron and phosphorus is not ideal, and it is difficult to separate Al 3+ and Fe 3+ in the leaching solution, which affects the preparation of iron phosphate. There is no reported idea and method in the existing technology to selectively produce an iron-phosphorus alloy from the lithium extraction residue of waste lithium iron phosphate and further regenerate it to prepare battery-grade iron phosphate. Summary of the Invention

[0009] Aiming at the technical gap in the existing technology that lacks the technology for resourcefully preparing an iron-phosphorus alloy from lithium iron phosphate lithium extraction residue and regenerating iron phosphate based on the iron-phosphorus alloy, the present invention provides a new method for preparing battery-grade iron phosphate from lithium iron phosphate lithium extraction residue, aiming to convert the lithium iron phosphate lithium extraction residue into the target phase iron-phosphorus alloy, and then chemically convert it to obtain high-quality battery-grade iron phosphate.

[0010] The present invention first proposes in the industry the idea of resourcefully converting lithium-extracted slag from lithium iron phosphate into ferrophosphorus alloy and further chemically converting the ferrophosphorus alloy to prepare iron phosphate. However, earlier studies have shown that to successfully implement this technical idea, many technical problems need to be overcome. For example, it is necessary to overcome the problems such as the difficulty in successfully achieving ferrophosphorus alloying and the low quality of alloying caused by the special physical and chemical characteristics of the lithium-extracted slag from lithium iron phosphate, and it is also necessary to overcome the problem of low quality of the iron phosphate prepared by resource utilization caused by the difficulty in controlling the phase of the ferrophosphorus alloy. In view of the special implementation problems faced by the idea described in the present invention, the present invention has conducted in-depth research and provides the following improvement measures;

[0011] A method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate, the steps including:

[0012] Step 1:

[0013] Mix the lithium-extracted slag of lithium iron phosphate and a binder to form pellets and perform heat treatment to obtain pretreated pellets;

[0014] Step 2:

[0015] Mix the pretreated pellets with a carbonaceous material, additive A, and additive B and perform heat preservation treatment under a reducing atmosphere at a temperature T1; remove slag and separate to obtain ferrophosphorus alloy;

[0016] The additive A includes silicon dioxide or a component that can decompose silicon dioxide during the heat preservation stage; the additive B includes a metal M oxide and a component that can decompose the metal M oxide during the heat preservation stage, wherein M includes at least one of Ca, Mg, and Al; the weight ratio of additive B to additive A is 1.0 - 2.0;

[0017] The temperature T1 is above 1550 °C;

[0018] Step 3:

[0019] Treat the ferrophosphorus alloy by acid treatment and roasting treatment to obtain battery-grade iron phosphate.

[0020] The present invention innovatively pre-treats the lithium-extracted slag of lithium iron phosphate by pelletizing treatment under the conditions such as binder assistance, and then performs heat preservation treatment with a carbonaceous material, additive A, and additive B under a reducing atmosphere and at the temperature T1. In this way, high-quality ferrophosphorus alloy can be prepared by resource utilization of the lithium-extracted slag based on a one-step fusion process. In addition, it is also beneficial to regulate the phase of the ferrophosphorus alloy to convert it highly selectively into a physical and chemical and crystal structure that is conducive to the synthesis of high-quality iron phosphate, thereby facilitating the subsequent resource utilization to prepare high-quality battery-grade iron phosphate.

[0021] In the present invention, the lithium-extracted residue of lithium iron phosphate is the residue after lithium extraction treatment of waste lithium iron phosphate cathode materials. The lithium extraction process can be conventional, for example, it can be water leaching, acid leaching, etc.

[0022] In the lithium-extracted residue of lithium iron phosphate, Fe, P, and C are the main occurring elements, and the contents are, for example, 50-55%, 30-35%, and 5-15% respectively. The total content of other impurity elements such as Al, F, Ti, etc. can be 0.5%-5%; Fe and P mainly exist in the FePO4 phase.

[0023] The particle size of the lithium-extracted residue of lithium iron phosphate is less than 200 mesh.

[0024] In the present invention, innovatively, the lithium-extracted residue of lithium iron phosphate and the binder are mixed and pelletized and then heat-treated to achieve pelletization. In this way, it can cooperate synergistically with the subsequent process, which is beneficial to the one-pot high-efficiency synthesis of ferrophosphorus alloy. In addition, it is also beneficial to improve the one-pot separation selectivity and effect of impurities and ferrophosphorus alloy, and is beneficial to regulating the phase structure characteristics of the phosphoric alloy, and further more beneficial to the subsequent preparation of high-quality battery-grade iron phosphate.

[0025] In the present invention, the binder includes bentonite and starch with a weight ratio of 0.5-2:1. Research in the present invention shows that under this preferred composite binder, synergy can be further achieved, which is more beneficial to adapting to the special physical and chemical characteristics of the lithium-extracted residue, and is more beneficial to obtaining an alloy with a Fe2P structure in the subsequent reaction. In this way, it helps to further improve the preparation of high-quality battery-grade iron phosphate.

[0026] The binder is 1-10% of the weight of the lithium-extracted residue of lithium iron phosphate; further, it can be 3-5%.

[0027] The diameter of the pelletized pellets is 5-20 mm.

[0028] The heat treatment temperature is 350-550 °C, and further it can be 400-500 °C.

[0029] The heat treatment time is 1-3 h.

[0030] In the present invention, the lithium-extracted residue of lithium iron phosphate is pelletized in advance and then heat-insulated and treated in combination with the carbonaceous material, additive A, and additive B under the reducing atmosphere and temperature T1. In this way, it is beneficial to the conversion of ferrophosphorus alloy and the regulation of its phase structure. In addition, it is also beneficial to the selective separation of impurities in the slag and ferrophosphorus alloy, and further beneficial to the subsequent preparation of high-quality battery-grade iron phosphate.

[0031] In the present invention, the carbonaceous material includes at least one of coke and charcoal.

[0032] The carbonaceous material is 5-20% of the weight of the pellets, and further it can be 10-15%.

[0033] In the present invention, the dosage of additive A is 1-3.5% of the weight of the pellets; further, it can be 2-3%. The weight ratio of additive B to additive A is 1.2-1.5:1.

[0034] In the present invention, the reducing atmosphere is an atmosphere containing at least one reducing gas among hydrogen, methane, and carbon monoxide; the content of the reducing gas therein is 5-30 v%, and further, it can be 10-20 v%.

[0035] The reducing atmosphere is also allowed to contain a diluent gas, such as at least one of nitrogen and inert gases.

[0036] In the present invention, the temperature of T1 is 1550-1700 °C.

[0037] In the present invention, the heat preservation time at temperature T1 is 1-4 h, and further, it can be 2-3 h.

[0038] In the present invention, in step 3, the ferrophosphorus alloy is placed in an acid solution for acid treatment.

[0039] The acid solution includes an acid solution containing at least one of sulfuric acid, hydrochloric acid, and nitric acid. There is no special requirement for the concentration of the solute in the acid solution. For example, it can be 0.1-2 M.

[0040] The pH at the reaction end point of the acid treatment is controlled at 1-3, and further, it can be 2-2.5.

[0041] In the present invention, after the acid treatment, solid-liquid separation is carried out. The solid is washed with water, dried, and then calcined to obtain the battery-grade iron phosphate.

[0042] Among them, the calcination temperature is 500-850 °C, and further, it can be 650-800 °C.

[0043] The calcination time can be 1-5 h, and further, it can be 2-4 h.

[0044] In the present invention, the obtained iron phosphate can be subjected to conventional lithium-doping roasting to obtain a lithium iron phosphate material. For example, the temperature of the lithium-doping roasting can be 700-800 °C, and the time can be 5-15 h. A small amount of conventional carbon source (such as sugars) is allowed to be added in the lithium-doping roasting stage, and its addition amount can be, for example, 5-15%.

[0045] Beneficial effects

[0046] The present invention first proposes a brand-new idea of converting waste lithium iron phosphate lithium extraction slag into ferrophosphorus alloy and resourcefully preparing battery-grade iron phosphate based on the chemical conversion of ferrophosphorus alloy.

[0047] In view of the problems faced by the inventive concept, the present invention innovatively pre-pelletizes the lithium-extracted slag, and then performs heat preservation treatment under the combined control of carbonaceous materials, additive A, additive B, reducing atmosphere and temperature. In this way, not only can iron-phosphorus alloy be effectively synthesized, but also the separation of impurities in the slag and the iron-phosphorus alloy with high selectivity is facilitated. Moreover, it can unexpectedly adjust the phase and physicochemical structure of the iron-phosphorus alloy, making it highly synergistically coupled with the subsequent chemical conversion process of the iron-phosphorus alloy, and improving the preparation effect of the battery-grade iron phosphate prepared.

[0048] The process flow of the present invention is relatively simple, easy to operate, and easy to realize industrial production. Each link in the process flow is closely connected, which can effectively improve production efficiency and product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is the technical route diagram of Example 1;

[0050] Figure 2 It is the XRD of the iron-phosphorus alloy obtained in Example 1;

[0051] Figure 3 It is the XRD of the iron phosphate obtained in Example 1;

[0052] Figure 4 It is the XRD of the lithium iron phosphate obtained in Example 1;

[0053] Figure 5 It is the first charge and discharge effect of the lithium iron phosphate obtained in Example 1 at 0.1C;

[0054] Figure 6 It is the XRD of the iron-phosphorus alloy obtained in Example 2;

[0055] Figure 7 It is the XRD of the final iron phosphate product obtained in Comparative Example 1;

[0056] Figure 8 It is the XRD of the iron phosphate product obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0057] The lithium-extracted slag of lithium iron phosphate described in the present invention can be the residue obtained after extracting lithium from waste lithium iron phosphate cathode by known means.

[0058] The method for preparing battery-grade iron phosphate from the lithium-extracted slag of lithium iron phosphate described in the present invention may include the following steps:

[0059] Step a. Pelletization treatment:

[0060] Crush, screen, mix with binder, pelletize, dry and preheat the lithium-extracted slag of lithium iron phosphate to obtain the pre-treated pellets;

[0061] Among them, particles with a particle size less than 200 mesh are obtained after crushing and screening; the binder is selected from bentonite, diatomaceous earth, starch or a mixture thereof, and the addition amount is 1-10% of the slag weight; the pellet diameter is 5-20 mm; the drying temperature is 100-200 °C; the preheating temperature is 350-550 °C.

[0062] Step b. Alloying:

[0063] Place the pretreated pellets in a reactor, add a carbonaceous material, additive A and additive B, and carry out a reduction reaction under a reducing atmosphere to remove the slag phase to obtain a high-purity ferrophosphorus alloy;

[0064] The carbonaceous material is selected from coke, charcoal or a mixture thereof; the carbonaceous material is 5-20% of the pellet weight;

[0065] Additive A is silica, and additive B includes at least one of calcium oxide, magnesium oxide, and aluminum oxide; the dosage of additive A is 1-3.5% of the pellet weight; the weight ratio of additive B / additive A is controlled at 1.0-2.0; and stirring is carried out, and the stirring speed is 10-50 r / min and the time is 10-30 min.

[0066] The temperature of the reduction reaction is 1550-1700 °C;

[0067] The reducing atmosphere is a mixed gas of an inert gas and a reducing gas. The inert gas is selected from argon, nitrogen or a mixture thereof, and the reducing gas is selected from hydrogen, methane, carbon monoxide or a mixed gas thereof. The volume fraction of the reducing gas is 5-30%; the time of the reduction reaction is controlled within 1-4 hours.

[0068] The atmosphere in the reactor is adjusted by controlling the flow rate ratio of the inert gas and the reducing gas. The flow rate of the inert gas is 1-10 m³ / h, and the flow rate of the reducing gas is 0.5-5 m³ / h. The reactor is selected from an electric arc furnace, an induction furnace or a resistance furnace.

[0069] Step c. Further prepare battery-grade iron phosphate from ferrophosphorus alloy:

[0070] Using the obtained high-purity ferrophosphorus alloy as a raw material, it is further processed by a chemical method to obtain battery-grade iron phosphate.

[0071] The chemical treatment method specifically includes: dissolving the high-purity ferrophosphorus alloy in an acid solution, and the acid solution is selected from sulfuric acid, hydrochloric acid or nitric acid; adjusting the pH value to 1-3; through precipitation, washing, drying and calcination treatment, the calcination temperature is 500-850 °C, to obtain battery-grade iron phosphate.

[0072] The high-purity ferrophosphorus alloy prepared by the method of the present invention has a phosphorus content of 15-25 wt%, an iron content of 75-85 wt%, and the total impurity content is less than 0.5 wt%, and has excellent processing performance and physical and chemical properties.

[0073] The battery-grade iron phosphate prepared by the method of the present invention has a particle size of 1-10 μm, a specific surface area of 10-50 m² / g, a tapped density of 1.0-2.0 g / cm³, a purity of over 99%, and excellent electrochemical properties.

[0074] In the present invention, the lithium-extracted slag of lithium iron phosphate is the residue after the lithium extraction treatment of the waste lithium iron phosphate cathode material. The main occurring elements in the lithium-extracted slag of lithium iron phosphate are Fe, P, and C, with contents of 50-55%, 30-35%, and 5-15% respectively, and the total content of other impurity elements such as Al, F, Ti, etc. is 0.5%-5%; Fe and P mainly exist in the FePO4 phase. The particle size of the lithium-extracted slag of lithium iron phosphate is less than 200 mesh.

[0075] Example 1:

[0076] Step a:

[0077] Take the lithium-extracted slag of lithium iron phosphate with element ratios of 55% Fe, 30% P, 12% C, and a total impurity content of 3%. Crush and screen the lithium-extracted slag of lithium iron phosphate to obtain particles with a particle size less than 200 mesh. Add 3% of the slag weight of bentonite as a binder, mix evenly and then pelletize, and the pellet diameter is 10 mm. Dry the formed pellets at 150°C for 2 hours, and then preheat at 400°C for 1 hour to obtain pellets.

[0078] Step b:

[0079] Place the pretreated pellets in an electric arc furnace, and control the furnace temperature at 1550°C. The furnace atmosphere is a mixed gas of argon and hydrogen, where the volume fraction of hydrogen is 10%, and the flow rate of the inert gas is 5 m³ / h. Add 10% of the pellet weight of coke as a carbonaceous material, and 2% of the slag weight of silica (additive A), and calcium oxide as additive B. Control the ratio of additive B / additive A at 1.2, turn on the electromagnetic stirring, the stirring speed is 30 r / min, keep stirring for 20 min, and the reduction reaction time is 2 hours to remove the impurities in the melt. After the refining is completed, cast the melt into a ferrophosphorus alloy ingot, and its XRD is shown in Figure 2 .

[0080] Step c:

[0081] Dissolve the high-purity ferrophosphorus alloy ingot in a 0.5-2M sulfuric acid solution, adjust the pH value to 2 to precipitate. Wash and dry the precipitate, and then calcine it at 700 °C for 2-3 hours to obtain battery-grade iron phosphate. The XRD of the material is shown in Figure 3 .

[0082] Step d:

[0083] Mix the iron phosphate prepared in step c and lithium carbonate according to the stoichiometric ratio, add 5% glucose and deionized water (the liquid-solid ratio can be 5-10 ml / g) based on the total weight of the total materials (the total weight of iron phosphate and lithium carbonate), stir evenly, and wet-mill the mixture. Usually, a sand mill is used for coarse grinding and fine grinding to obtain precursor powder with a particle size of 1 μm. Pre-burn the precursor powder at 350 °C for 5 hours, and then keep it at 800 °C for 10 hours under the protection of an inert nitrogen atmosphere to complete the reaction. After cooling, lithium iron phosphate is obtained. The XRD of this material is shown in Figure 4 .

[0084] Test results: The main phase in the ferrophosphorus alloy is Fe2P, accounting for 82% of the total amount, accompanied by the FeP phase, accounting for 18% of the total amount. The phosphorus content in the alloy is 21 wt%, the iron content is 78 wt%, the total impurity content is less than 0.3 wt%, and the recovery rate of Fe reaches 97.5%. The particle size of the battery-grade iron phosphate is 5 μm, the specific surface area is 25 m² / g, the tap density is 1.5 g / cm³, and the purity reaches 99.5%. The first charge-discharge specific capacity of lithium iron phosphate at 0.1C reaches 152 mA·h. The electrochemical test diagram is shown in Figure 5 .

[0085] Example 2:

[0086] Step a:

[0087] Take the lithium iron phosphate lithium extraction slag with element ratios of 53% Fe, 32% P, 10% C, and a total impurity of 5%. Crush and screen the slag to a particle size <120 mesh, add 5% diatomaceous earth binder based on the weight of the slag, and mix evenly to form pellets (pellet diameter 22 mm). Dry the pellets at 200 °C for 5 hours, and then preheat at 500 °C for 2.5 hours.

[0088] Step b:

[0089] Place the pellets in an electric resistance furnace with a furnace temperature of 1650 °C. Pass a mixed gas of argon and methane (methane volume fraction 18%) into the furnace, and the flow rate of the inert gas is 12 m³ / h. Add 15% coke powder based on the weight of the pellets, 3% silica (additive A) and calcium oxide (additive B) based on the weight of the slag, and control the ratio of additive B / additive A to be 1.5. Turn on the electromagnetic stirring (45 r / min, 40 min), and after the reduction reaction for 3 hours, cast it into a ferrophosphorus alloy ingot. The XRD is shown inFigure 6 。

[0090] Step c:

[0091] Dissolve the ferrophosphorus alloy ingot in a 0.5 - 1 M nitric acid solution, adjust the pH to 3.0 to precipitate. After washing and drying the precipitate, calcine it at 800 °C for 2 - 2.5 h to obtain battery-grade iron phosphate.

[0092] Mix the prepared iron phosphate and lithium carbonate in a stoichiometric ratio, add 8% glucose of the total material mass and an appropriate amount of deionized water, stir evenly, wet-mill the mixture, usually using a sand mill for coarse grinding and fine grinding, control the particle size at 1 μm, pre-calcine the precursor powder at 300 °C for 4 hours to preliminarily decompose the raw materials, under the protection of an inert nitrogen atmosphere, heat the pre-calcined precursor to 700 °C, hold the temperature for 8 hours to complete the reaction, and obtain lithium iron phosphate after cooling.

[0093] Test results: In the ferrophosphorus alloy, the main phase is Fe2P, accounting for 84% of the total amount, and there is a small amount of FeP phase present, accounting for 16% of the total amount. The recovery rate of Fe reaches 98.3%. Compared with Example 1, there is a partial transformation from FeP to Fe2P. The phosphorus content is 23 wt%, the iron content is 76 wt%, and the total impurity content is < 0.4 wt%. The particle size of the battery-grade iron phosphate is 8 μm, the specific surface area is 30 m² / g, the tapped density is 1.6 g / cm³, and the purity reaches 99.7%; the initial charge-discharge specific capacity of lithium iron phosphate at 0.1C reaches 147.8 mA·h.

[0094] Example 3

[0095] Compared with Example 1, the difference is only that the binder in Step a is changed, and other operations and parameters are the same as in Example 1. The experimental groups are as follows:

[0096] Group A: The binder is starch, and its dosage is the same as in Example 1.

[0097] Group B: The binder is bentonite and starch with a weight ratio of 1:1, and the total dosage of the binder is the same as in Example 1.

[0098] The results are as follows:

[0099] Group A: The iron recovery rate of the slag is 95.8%. In the ferrophosphorus alloy, the main phase is Fe2P, with a content of about 81%, and the total impurity content is less than 0.4 wt%. The particle size of the battery-grade iron phosphate is 7 μm, the purity reaches 99.5%, and the initial charge-discharge specific capacity of lithium iron phosphate at 0.1C reaches 150 mA·h.

[0100] Group B: The iron recovery rate of the slag is 98.9%. The Fe2P phase of ferrophosphorus alloy has a content of about 88%, and the total impurity content is less than 0.3 wt%. The particle size of battery-grade iron phosphate is 4.3 μm, the purity reaches 99.6%, and the initial charge-discharge specific capacity of lithium iron phosphate at 0.1C reaches 154 mA·h.

[0101] Research shows that through the bentonite / starch composite binder, the spheroidization morphology of the slag can be optimized, which is beneficial to subsequent combined reduction, and can synergistically improve the Fe2P phase of ferrophosphorus alloy, thereby facilitating the subsequent preparation of lithium iron phosphate materials with excellent regeneration activity.

[0102] Comparative Example 1

[0103] Compared with Example 1, the difference is only that the slag is not pelletized in step a, but the lithium-extracted slag and the binder are directly subjected to step b and subsequent treatments, and other operations and parameters are the same as those in Example 1. The XRD of the finally prepared iron phosphate material is shown in Figure 7 . From Figure 7 it can be seen that FePO4 in the lithium-extracted slag of lithium iron phosphate is not completely decomposed and reduced, and the final product consists of ferrophosphorus alloy and part of FePO4, and battery-grade iron phosphate cannot be obtained.

[0104] Comparative Example 2

[0105] Compared with Example 1, the difference is only that the furnace temperature in step b is controlled at 1500 °C, and other operations and parameters are the same as those in Example 1.

[0106] The XRD of the finally prepared iron phosphate material is shown in Figure 8 . From Figure 8 it can be seen that FePO4 in the lithium-extracted slag of lithium iron phosphate is not completely decomposed and reduced, and the final product consists of ferrophosphorus alloy and part of FePO4, and battery-grade iron phosphate cannot be obtained.

[0107] Comparative Example 3

[0108] Compared with Example 1, the difference is only that additive B is not added, and the dosage of additive A is the same as the total weight of additive A + B in Example 1, and other operations and parameters are the same as those in Example 1.

[0109] Test effect: The P content in ferrophosphorus alloy is only 18 wt%, and the proportion of FeP phase > 40%; the FeO content in the molten slag reaches 8.2%, and the iron recovery rate drops to 88% (≥95% in Example 1).

[0110] Comparative Example 4

[0111] Compared with Example 1, the difference is only that in step a, the binder is changed to water glass (Na2SiO3, addition amount 3%). Other operations and parameters are the same as those in Example 1.

[0112] Test results: The compressive strength of the green balls is only 80 N per ball, and the cracking rate during the drying process is > 30%; the iron recovery rate after the reduction of the pellets is 91% (96.5% in Example 1), and the reaction contact area decreases due to the pulverization of the pellets.

Claims

1. A method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate, characterized in that the steps Including: Step 1: Mix the lithium-extracted slag of lithium iron phosphate and a binder, pelletize and heat-treat to obtain a pretreated pellet; the binder is at least one of bentonite, diatomite, and starch; the binder is 1-10% by weight of the lithium-extracted slag of lithium iron phosphate; the heat-treatment temperature is 350-550 °C. Step 2: Mix the pretreated pellet with a carbonaceous material, additive A, and additive B, and carry out heat preservation treatment under a reducing atmosphere and at temperature T1. Remove slag and separate to obtain ferrophosphorus alloy. The additive A is silicon dioxide; the additive B is CaO; the weight ratio of additive B to additive A is 1.2-1.5:1; the dosage of additive A is 1-3.5% by weight of the pellet; the reducing atmosphere is an atmosphere containing at least one reducing gas of hydrogen, methane, and carbon monoxide; the content of the reducing gas therein is 5-30 v%. The temperature T1 is 1550-1700 °C. Step 3: Carry out acid treatment and roasting treatment on the ferrophosphorus alloy to obtain battery-grade iron phosphate.

2. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein The lithium-extracted slag of lithium iron phosphate is the residue after lithium extraction treatment of waste lithium iron phosphate cathode material. The particle size of the lithium-extracted slag of lithium iron phosphate is less than 200 mesh.

3. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein The diameter of the pelletized pellet is 5-20 mm.

4. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein, The carbonaceous material includes at least one of coke and charcoal. The carbonaceous material is 5-20% by weight of the pellet.

5. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein, The dosage of additive A is 2-3% by weight of the pellet.

6. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein The reducing atmosphere is an atmosphere containing at least one reducing gas of hydrogen, methane, and carbon monoxide; the volume fraction of the reducing gas therein is 10-20 v%.

7. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein The binder includes bentonite and starch with a weight ratio of 0.5-2:

1.

8. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein, The heat preservation time at temperature T1 is 1-4 h.

9. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein In Step 3, place the ferrophosphorus alloy in an acid solution for acid treatment. The acid solution includes an acid solution of at least one of sulfuric acid, hydrochloric acid, and nitric acid. The pH at the reaction end point of the acid treatment is controlled at 1-3.

10. The method for preparing battery-grade iron phosphate from lithium-extracted slag of lithium iron phosphate as claimed in claim 1, wherein After acid treatment, carry out solid-liquid separation, wash the solid with water, dry it, and then calcine to obtain the battery-grade iron phosphate. Among them, the calcination temperature is 500-850 °C.

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