CN116443838A - A method for cascade utilization of waste lithium iron phosphate cathode powder - Google Patents

Abstract

The invention provides a method for cascade utilization of waste lithium iron phosphate anode powder, which comprises the following steps: 1) Carrying out gradient heating roasting on waste lithium iron phosphate anode powder in an inert atmosphere; 2) Vibrating, screening and color selecting the roasted product; 3) Floating the undersize to obtain lithium iron phosphate anode powder and graphite respectively; 4) Adding acid into the oversize material for leaching; 5) Regulating the pH value of the leaching solution, and adding an oxidant and a precipitator to precipitate iron and phosphorus; 6) Adding sodium hydroxide into the solution after precipitation to further precipitate aluminum in the solution; 7) Extracting, separating and recycling copper, nickel and cobalt in the filtrate in the step 6); 8) Adding sodium carbonate into the extracted solution to obtain lithium acid precipitate, and evaporating and crystallizing the solution to obtain sodium salt. The invention can greatly reduce the waste of the lithium iron phosphate battery, realize the cascade utilization of the waste lithium iron phosphate positive electrode powder, and avoid the problems of incomplete recovery and large pollution caused by completely using a pyrogenic process or a wet process.

Description

A method for cascade utilization of waste lithium iron phosphate cathode powder

technical field

The invention belongs to the technical field of recycling and comprehensive utilization of waste lithium-ion power batteries. Specifically, the invention relates to a method for cascade utilization of waste lithium iron phosphate cathode powder.

Background technique

With the rapid development of the national economy, non-renewable energy represented by oil, coal, and natural gas is gradually decreasing, and the rapid development of the new energy industry provides unprecedented opportunities for the application of lithium-ion batteries in the field of energy storage batteries. However, the lifespan of lithium-ion batteries is generally 2-5 years. With the widespread use of lithium-ion batteries, a large number of waste lithium-ion batteries will be produced. If it is not recycled, it will not only cause a serious waste of resources, but also cause serious environmental pollution.

Lithium-ion power batteries with lithium iron phosphate as the positive electrode material have been widely used in the energy storage industry due to their low cost, good cycle performance, and good safety performance. In 2021, the installed capacity of lithium iron phosphate batteries will exceed that of ternary lithium batteries. It can be expected that as these batches of lithium iron phosphate battery materials are scrapped, more and more waste lithium iron phosphate batteries will be produced. Therefore, it is particularly important to recycle lithium-ion batteries with lithium iron phosphate as the positive electrode.

The waste positive lithium iron phosphate is black powder. According to the different sorting efficiency of the dismantling equipment, the waste positive electrode powder contains aluminum, copper, fluorine, binder, electrolyte/electrolyte, and due to sorting reasons, some waste lithium iron phosphate positive electrodes There is also a small amount of ternary cathode material in the powder. Lithium iron phosphate, as the positive electrode active material of the power battery, has an olivine structure and a very stable crystal structure. When lithium ions are inserted and moved out, the crystal structure changes little, which determines its long cycle life. Both A123 and Valance believe that the failure of lithium iron phosphate batteries is mainly due to the failure of the structure of the negative electrode active material, which provides strong support for the direct recycling of lithium iron phosphate cathode powder.

At present, the recovery process of waste lithium iron phosphate cathode powder is mainly divided into fire method and wet method. The fire method is mainly high-temperature solid-phase regeneration method. Incomplete utilization of elements; wet process consumes a lot of acid, high cost, and there are also problems such as incomplete utilization of useful elements.

Chinese patent CN201510372381.0 discloses a method for recycling battery-grade iron phosphate from lithium iron phosphate batteries and using waste lithium iron phosphate batteries to prepare lithium iron phosphate cathode materials. The lithium iron phosphate cathode materials are pulverized and heat-treated, acid leached and alkali is added Adjust the pH to recover iron phosphate, then add alkali and sodium carbonate to prepare lithium carbonate, and finally reduce iron phosphate, sodium carbonate and carbon powder to short-fire to obtain lithium iron phosphate cathode material. This patent leaches all lithium iron phosphate, which consumes a lot of acid, and some impurities such as PVDF and graphite powder are not fully recycled.

Chinese patent CN102208707 (a method for repairing and regenerating the positive electrode material of waste lithium iron phosphate battery) and Chinese patent CN102208706A (a method for recycling and regenerating the positive electrode material of waste lithium iron phosphate battery) use a simple calcination method to remove carbon, conductive agent and sticky material. The binder is not economical and consumes a lot of energy, and it is necessary to further add iron lithium and reducing agents to the iron lithium material in order to be further reused.

Therefore, in order to solve the problem of comprehensive recycling of waste lithium iron phosphate positive electrode powder, it is necessary to seek a new recycling method for waste lithium iron phosphate positive electrode powder, so that all valuable elements of waste lithium iron phosphate positive electrode powder can be recovered to avoid polluting the environment .

Contents of the invention

In view of the shortcomings of the existing technology, the present invention provides a method for cascaded utilization of waste lithium iron phosphate positive electrode powder, which can greatly reduce the waste of lithium iron phosphate batteries, realize cascade utilization of waste lithium iron phosphate positive electrode powder, and avoid complete use The problems of incomplete recovery and high pollution caused by pyrotechnic or wet process.

In order to achieve the above object, the present invention has adopted following technical scheme:

A method for cascade utilization of waste lithium iron phosphate positive electrode powder, said method comprising the steps of:

1) Roasting the waste lithium iron phosphate cathode powder after discharging, dismantling, crushing and sorting in an inert atmosphere with gradient temperature increase;

2) Sieving or color sorting the roasted product;

3) Carry out flotation on the undersieve to obtain a concentrate rich in lithium iron phosphate positive electrode powder and tailings rich in graphite, and the concentrate rich in lithium iron phosphate positive electrode powder can be reused as a battery material;

4) The tailings rich in graphite and the sieve are mixed and subjected to step-by-step acid leaching, one-step acid leaching is added with dilute acid, one-step acid leaching residue is subjected to two-step acid leaching, two-step acid leaching is added with concentrated acid, and two-step acid leaching residue is After washing and drying, high-purity graphite is obtained; the washing solution is reused for one-step acid leaching and two-step acid leaching, and the second-step acid leaching solution is used for one-step acid leaching;

5) adjust the pH value of the one-step pickling solution, and add a certain amount of oxidant and precipitant to precipitate iron and phosphorus in the one-step pickling solution;

6) adding sodium hydroxide to the precipitated liquid to further precipitate the aluminum therein, and filtering to obtain the aluminum salt;

7) The filtrate in step 6) is extracted and separated to obtain a copper-nickel-cobalt product;

8) Add sodium carbonate to the extracted liquid, filter to obtain lithium carbonate precipitate, and evaporate and crystallize the filtrate to obtain sodium salt.

Preferably, the gradient temperature-increasing calcination in step 1) is segmented calcination, the calcination temperature of the first stage is less than 200°C, and the calcination temperature of the second stage is 400-700°C.

Preferably, the dilute acid and the concentrated acid used in step 4) are sulfuric acid, nitric acid or hydrochloric acid.

Preferably, in step 4), the dilute acid concentration of the one-step acid leaching is 5%-20%, and the reaction temperature is 20-60°C.

Preferably, in step 4), the concentrated acid concentration of the two-step acid leaching is 20%-60%, and the reaction temperature is 80-150°C.

Preferably, in step 5), the oxidant is one or more of air, oxygen, ozone, hydrogen peroxide and sodium pyrosulfate.

Preferably, in step 5), the pH is adjusted to 1.5-2, the material for adjusting the pH is 5-50% sodium hydroxide or sodium carbonate, and the pH adjustment time is 60-180 minutes.

Preferably, in step 5), the precipitation agent is the iron salt of the corresponding leaching acid, and the amount of the iron salt added is 1.01-1.03 times the iron content in the solution.

According to a preferred embodiment of the present invention, a method for cascade utilization of waste lithium iron phosphate cathode powder comprises the following steps:

1) The waste lithium iron phosphate positive electrode powder that has been discharged, disassembled, crushed, and sorted is roasted in an inert atmosphere with a gradient temperature rise, and a small part of the attached electrolyte is recovered and the binder is removed in depth to destroy the graphite and The mutual bonding state of lithium iron phosphate improves the flotation effect;

2) Carry out vibration screening and color sorting to the roasted product, so that copper, aluminum and large particles are initially separated from the lithium iron phosphate powder attached thereto;

3) Carry out flotation on the undersieve to obtain a concentrate rich in lithium iron phosphate positive electrode powder and a tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

4) A certain amount of lithium iron phosphate remains in graphite, copper flakes, aluminum flakes, and the sieve, acid leaching step by step, and dilute acid is added in the first step to make phosphorus, iron, lithium, copper, aluminum, nickel, and cobalt enter the solution , separated from graphite. In the second step, concentrated acid is added for further leaching, and the leaching solution is recycled. After reaching a certain concentration, the acid leaching solution is used for the first step of acid leaching, and the acid leaching residue is washed and dried to prepare battery materials;

5) adjusting the pH value of the first step leaching solution in step 4), and adding a certain amount of oxidizing agent and precipitant to make iron and phosphorus basically completely precipitate;

6) adding sodium hydroxide to the precipitated liquid to further precipitate the aluminum therein;

7) extracting and separating the filtrate in 6), and recovering copper, nickel and cobalt therein respectively;

8) Add sodium carbonate to the extracted solution to obtain lithium acid precipitate, and evaporate and crystallize the solution to obtain sodium salt.

"%" in the present invention refers to mass percentage unless otherwise specified.

Compared with prior art, the beneficial effect of the present invention is:

(1) Comprehensive utilization of all components, including a small amount of electrolyte, PVDF and a small amount of phosphorus in the solution, to reduce environmental pollution;

(2) The method provided by the present invention can partially recover the lithium iron phosphate that has not reached the cycle life, and can greatly reduce the cost of follow-up treatment;

(3) The two-step acid leaching recovers iron phosphate, lithium carbonate, copper, aluminum and high-purity graphite, which greatly improves resource utilization and product quality, and has obvious social and economic benefits.

Description of drawings

Fig. 1 is a flow chart of a method for cascaded utilization of waste lithium iron phosphate cathode powder according to the present invention.

Detailed ways

The present invention will be further described in detail with the accompanying drawings and specific embodiments.

Example 1

As shown in Figure 1, a method for cascade utilization of waste lithium iron phosphate cathode powder, the method shown includes the following steps:

(1) The waste lithium iron phosphate positive electrode powder that has been discharged, disassembled, crushed, and sorted is roasted at 155°C for 50 minutes in an argon atmosphere, and the organic solvent therein is recovered. Further heat up to 650°C and bake for 30 minutes to decompose the binder in it, so that graphite and lithium iron phosphate can be separated;

(2) Carry out vibratory screening to the product after roasting, separate the large particles such as copper flakes and aluminum flakes from the small particles, the oversize is removed for acid leaching, and the undersize is used for flotation;

(3) Carry out flotation to the underscreen, obtain the concentrate rich in lithium iron phosphate positive electrode powder and the tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

(4) Add graphite-rich tailings, graphite, copper flakes, aluminum flakes, and sieves to the acid leaching reactor, and start the first step of acid leaching. This step is acid leaching with 10% dilute sulfuric acid, and the reaction temperature is 40 ℃, the reaction time is 60 minutes, so that phosphorus, iron, lithium, copper, aluminum, nickel, and cobalt enter the solution and separate from graphite. After the above slurry is filtered and separated, the filter cake is subjected to the second step of acid leaching. In this step, 40% concentrated sulfuric acid is added, the reaction temperature is 80°C, and the reaction time is 60 minutes to further remove a small amount of impurity elements. The second step of acid leaching residue After washing and drying, high-purity graphite is obtained;

(5) Add ferric sulfate of 1.01 times of iron amount in the solution to the acid leaching solution obtained in the first step in (4), and add sodium pyrosulfate, then adjust the pH value to 1.5 with 20% sodium hydroxide, and adjust the pH time to 120 Minutes, so that iron and phosphorus are basically completely precipitated;

(6) adding sodium hydroxide to the precipitated liquid to adjust the pH to 5.0, further precipitating the aluminum therein;

(7) extracting and separating the filtrate in (6), reclaiming copper, nickel and cobalt therein respectively;

(8) Add sodium carbonate to the extracted liquid to obtain lithium carbonate precipitate, filter, evaporate and crystallize the filtrate to obtain sodium sulfate.

In this example, the recycling rate of the lithium iron phosphate positive electrode powder is 82%, the recovered lithium iron phosphate has a battery specific capacity of 131.65mAh/g at a charge and discharge rate of 0.2C, and the purity of the recovered graphite reaches 99.90%.

Example 2

As shown in Figure 1, a method for cascade utilization of waste lithium iron phosphate cathode powder, the method shown includes the following steps:

(1) The waste lithium iron phosphate positive electrode powder that has been discharged, disassembled, crushed, and sorted is roasted at 120° C. for 60 minutes in a nitrogen atmosphere, and the organic solvent therein is recovered. Further heat up to 450°C and bake for 50 minutes to decompose the binder in it, so that graphite and lithium iron phosphate can be separated;

(2) Carry out color sorting to the product after roasting, copper flake, aluminum flake are separated with spent positive electrode powder, copper and aluminum flake are removed acid leaching, and the rest is used for flotation;

(3) Carry out flotation to the underscreen, obtain the concentrate rich in lithium iron phosphate positive electrode powder and the tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

(4) Graphite-rich tailings, graphite, copper flakes, aluminum flakes, and color sorting materials are added to the acid leaching reaction kettle together, and the first step of acid leaching is started. In this step, 20% dilute sulfuric acid is added and passed through microbubbles The method is to feed air, the reaction temperature is 30°C, and the reaction time is 50 minutes, so that phosphorus, iron, lithium, copper, aluminum, nickel, and cobalt enter the solution and separate from graphite. After the above slurry is filtered and separated, the filter cake is subjected to the second step of acid leaching. In this step, 30% concentrated sulfuric acid is added, the reaction temperature is 100°C, and the reaction time is 60 minutes to further remove a small amount of impurity elements. The second step of acid leaching residue After washing and drying, high-purity graphite is obtained;

(5) Add ferric sulfate of 1.01 times the amount of iron in the solution to the acid leach solution obtained in the first step in (4), and feed air through the microbubble mode, and simultaneously adjust the pH value to 1.5 with 40% sodium hydroxide to adjust the pH The time is 160 minutes, so that iron and phosphorus are basically completely precipitated;

(6) adding sodium hydroxide to the precipitated liquid to adjust the pH to 4.9, further precipitating the aluminum therein;

(7) extracting and separating the filtrate in (6), reclaiming copper, nickel and cobalt therein respectively;

(8) Add sodium carbonate to the extracted liquid to obtain lithium carbonate precipitate, filter, evaporate and crystallize the filtrate to obtain sodium sulfate.

In this example, the recycling rate of the lithium iron phosphate cathode powder was 81%, the recovered lithium iron phosphate had a battery specific capacity of 126.51mAh/g at a charge-discharge rate of 0.2C, and the purity of the recovered graphite reached 99.91%.

Example 3

As shown in Figure 1, a method for cascade utilization of waste lithium iron phosphate cathode powder, the method shown includes the following steps:

(1) The waste lithium iron phosphate positive electrode powder that has been discharged, disassembled, crushed, and sorted is roasted at 185°C for 30 minutes in an argon atmosphere, and the organic solvent therein is recovered. Further heat up to 550°C and bake for 90 minutes to decompose the binder in it, so that graphite and lithium iron phosphate can be separated;

(2) Carry out vibratory screening to the product after roasting, separate the large particles such as copper flakes and aluminum flakes from the small particles, the oversize is removed for acid leaching, and the undersize is used for flotation;

(3) Carry out flotation to the underscreen, obtain the concentrate rich in lithium iron phosphate positive electrode powder and the tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

(4) Add graphite-rich tailings, graphite, copper flakes, aluminum flakes, and sieves to the acid leaching reactor, and start the first step of acid leaching. This step is acid leaching with 5% dilute hydrochloric acid, and the reaction temperature is 60 ℃, the reaction time is 60 minutes, so that phosphorus, iron, lithium, copper, aluminum, nickel, and cobalt enter the solution and separate from graphite. After the above slurry is filtered and separated, the filter cake is subjected to the second step of acid leaching. In this step, 31% concentrated hydrochloric acid is added, the reaction temperature is 150°C, and the reaction time is 60 minutes to further remove a small amount of impurity elements. After washing and drying, high-purity graphite is obtained;

(5) Add ferric sulfate of 1.02 times of iron amount in the solution to the acid leaching solution that the first step in (4) obtains, and pass into ozone by microbubble mode, adjust pH value to 1.8 with 10% sodium hydroxide simultaneously, adjust pH The time is 80 minutes, so that iron and phosphorus are basically completely precipitated;

(6) adding sodium hydroxide to the precipitated liquid to adjust the pH to 5.0, further precipitating the aluminum therein;

(7) extracting and separating the filtrate in (6), reclaiming copper, nickel and cobalt therein respectively;

(8) Sodium carbonate is added to the extracted liquid to obtain lithium carbonate precipitation, and the solution is evaporated and crystallized to obtain sodium chloride.

In this example, the recycling rate of lithium iron phosphate positive electrode powder is 85%, the recovered lithium iron phosphate has a battery specific capacity of 133.27mAh/g at a charge and discharge rate of 0.2C, and the purity of recovered graphite reaches 99.94%.

Example 4

As shown in Figure 1, a method for cascade utilization of waste lithium iron phosphate cathode powder, the method shown includes the following steps:

(1) Baking the waste lithium iron phosphate positive electrode powder after discharging, dismantling, crushing and sorting at 115° C. for 90 minutes in a nitrogen atmosphere, and recovering the organic solvent therein. Further heat up to 650°C and bake for 50 minutes to decompose the binder in it, so that graphite and lithium iron phosphate can be separated;

(2) Carry out vibratory screening to the product after roasting, separate the large particles such as copper flakes and aluminum flakes from the small particles, the oversize is removed for acid leaching, and the undersize is used for flotation;

(3) Carry out flotation to the underscreen, obtain the concentrate rich in lithium iron phosphate positive electrode powder and the tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

(4) Graphite-rich tailings, graphite, copper flakes, aluminum flakes, and sieves are added to the acid leaching reaction kettle together, and the first step of acid leaching is started. In this step, 10% dilute nitric acid is added and passed through microbubbles. Method, reaction temperature 30°C, reaction time 60 minutes, make phosphorus, iron, lithium and copper, aluminum, nickel, cobalt enter the solution and separate from graphite. After the above slurry is filtered and separated, the filter cake is subjected to the second step of acid leaching. In this step, 50% concentrated nitric acid is added, the reaction temperature is 100°C, and the reaction time is 60 minutes to further remove a small amount of impurity elements. The second step of acid leaching residue After washing and drying, high-purity graphite is obtained;

(5) Add ferric nitrate of 1.03 times of iron amount in the solution to the acid leaching liquid that the first step obtains in (4), and pass into the mixed gas of air and oxygen, regulate pH value to 2.0 with 5% sodium hydroxide simultaneously, adjust The pH time is 60 minutes, so that the iron and phosphorus are basically completely precipitated;

(6) adding sodium hydroxide to the precipitated liquid to adjust the pH to 5.1, further precipitating the aluminum therein;

(7) extracting and separating the filtrate in (6), reclaiming copper, nickel and cobalt therein respectively;

(8) Sodium carbonate is added to the extracted liquid to obtain lithium carbonate precipitation, and the solution is evaporated and crystallized to obtain sodium sulfate.

In this example, the recycling rate of lithium iron phosphate positive electrode powder is 86%, the recovered lithium iron phosphate has a battery specific capacity of 130.37mAh/g at a charge and discharge rate of 0.2C, and the purity of recovered graphite reaches 99.93%.

Example 5

As shown in Figure 1, a method for cascade utilization of waste lithium iron phosphate cathode powder, the method shown includes the following steps:

(1) The waste lithium iron phosphate positive electrode powder that has been discharged, disassembled, crushed, and sorted is roasted at 150° C. for 40 minutes in an atmosphere of argon and nitrogen gas mixture, and the organic solvent therein is recovered. Further heat up to 600°C and bake for 60 minutes to decompose the binder in it, so that graphite and lithium iron phosphate can be separated;

(2) For advanced color sorting of the roasted products, after the color sorting, further vibrating screening is carried out to separate large particles such as copper flakes and aluminum flakes from small particles. for flotation;

(3) Carry out flotation to the underscreen, obtain the concentrate rich in lithium iron phosphate positive electrode powder and the tailings rich in graphite, and the lithium iron phosphate positive electrode powder is reused as a battery;

(4) Add graphite-rich tailings, graphite, copper flakes, aluminum flakes, and color selection materials to the acid leaching reaction kettle together, and start the first step of acid leaching, and add 10% dilute hydrochloric acid to this step of acid leaching, The reaction temperature is 40°C, and the reaction time is 60 minutes, so that phosphorus, iron, lithium, copper, aluminum, nickel, and cobalt enter the solution and are separated from graphite. After the above slurry is filtered and separated, the filter cake is subjected to the second step of acid leaching. In this step, 31% concentrated hydrochloric acid is added, the reaction temperature is 110°C, and the reaction time is 80 minutes to further remove a small amount of impurity elements. After washing and drying, high-purity graphite is obtained;

(5) Add ferric chloride of 1.01 times the amount of iron in the solution to the acid leaching solution obtained in the first step in (4), and feed oxygen through the microbubble mode, and adjust the pH value to 2.0 with 30% sodium carbonate simultaneously to adjust the pH The time is 120 minutes, so that iron and phosphorus are basically completely precipitated;

(6) adding sodium hydroxide to the precipitated liquid to adjust the pH to 5.0, further precipitating the aluminum therein;

(7) extracting and separating the filtrate in (6), reclaiming copper, nickel and cobalt therein respectively;

(8) Sodium carbonate is added to the extracted liquid to obtain lithium carbonate precipitation, and the solution is evaporated and crystallized to obtain sodium chloride.

In this example, the recycling rate of lithium iron phosphate positive electrode powder is 90%, the recovered lithium iron phosphate has a battery specific capacity of 133.78mAh/g at a charge and discharge rate of 0.2C, and the purity of recovered graphite reaches 99.94%.

The process parameters (such as temperature, time, etc.) interval upper and lower limits and interval values of the present invention can realize this method, and the embodiments are not listed one by one here.

The conventional technical knowledge in this field can be used for the contents not described in detail in the present invention.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all should be covered by the present invention. within the scope of the claims.

Claims (8)

1. A method for cascade utilization of waste lithium iron phosphate cathode powder, said method comprising the steps of: 1) Roasting the waste lithium iron phosphate cathode powder after discharging, dismantling, crushing and sorting in an inert atmosphere with gradient temperature increase; 2) Sieving or color sorting the roasted product; 3) Carry out flotation on the undersieve to obtain a concentrate rich in lithium iron phosphate positive electrode powder and tailings rich in graphite, and the concentrate rich in lithium iron phosphate positive electrode powder can be reused as a battery material; 4) The tailings rich in graphite and the sieve are mixed and subjected to step-by-step acid leaching, one-step acid leaching is added with dilute acid, one-step acid leaching residue is subjected to two-step acid leaching, two-step acid leaching is added with concentrated acid, and two-step acid leaching residue is After washing and drying, high-purity graphite is obtained; the washing solution is reused for one-step acid leaching and two-step acid leaching, and the second-step acid leaching solution is used for one-step acid leaching; 5) adjust the pH value of the one-step pickling solution, and add a certain amount of oxidant and precipitant to precipitate iron and phosphorus in the one-step pickling solution; 6) adding sodium hydroxide to the precipitated liquid to further precipitate the aluminum therein, and filtering to obtain the aluminum salt; 7) The filtrate in step 6) is extracted and separated to obtain a copper-nickel-cobalt product; 8) Add sodium carbonate to the extracted liquid, filter to obtain lithium carbonate precipitate, and evaporate and crystallize the filtrate to obtain sodium salt. 2. The method for cascaded utilization of waste lithium iron phosphate positive electrode powder according to claim 1, characterized in that the gradient temperature-increasing roasting in step 1) is segmental roasting, the first-stage roasting temperature is less than 200°C, and the second-stage roasting temperature is less than 200°C. It is 400-700°C. 3. The method for cascade utilization of waste lithium iron phosphate positive electrode powder according to claim 1, characterized in that the acid used in the dilute acid and concentrated acid in step 4) is sulfuric acid, nitric acid or hydrochloric acid. 4. The method for cascade utilization of waste lithium iron phosphate cathode powder according to claim 1, characterized in that, in step 4), the dilute acid concentration of one-step acid leaching is 5%-20%, and the reaction temperature is 20-60°C. 5. The method for cascade utilization of waste lithium iron phosphate cathode powder according to claim 1, characterized in that, in step 4), the concentrated acid concentration of the two-step acid leaching is 20%-60%, and the reaction temperature is 80-150°C . 6. The method for cascade utilization of waste lithium iron phosphate cathode powder according to claim 1, wherein in step 5), the oxidizing agent is one or both of air, oxygen, ozone, hydrogen peroxide and sodium pyrosulfate more than one species. 7. The method for cascade utilization of waste lithium iron phosphate cathode powder according to claim 1, characterized in that, in step 5), the pH value is adjusted to 1.5-2, and the material for adjusting the pH value is 5-50% sodium hydroxide or Sodium carbonate, the pH adjustment time is 60-180 minutes. 8. The method for cascade utilization of waste lithium iron phosphate cathode powder according to claim 1, characterized in that, in step 5), the precipitant is the iron salt of the corresponding leaching acid, and the added amount of the iron salt is iron in the solution 1.01-1.03 times of content.