Iron phosphate calcination section tail powder recycling process

Abstract

The application relates to the field of lithium battery positive electrode material preparation, and specifically discloses a phosphorus acid iron calcination section tail powder recycling process, which comprises the following steps: (1) using five-stage filtrate to reslurry tailings; (2) conveying the well-stirred tailing slurry to a aging buffer tank to be warmed and aged; (3) centrifuging the aged slurry to separate out filter cake; (4) reslurrying the tailing filter cake and product slurry aged in the original production system to carry out secondary washing; and (5) after centrifuging the mixture, the mixture is subjected to drying and calcination process treatment to obtain a target product. The scheme can solve the problems that tailings after high-temperature aging are difficult to be recycled in the phosphorus acid iron system, and the product after recycling has high sulfur content and low specific surface.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode material preparation, specifically to a process for recycling tailings powder from the calcination of iron phosphate. Background Technology

[0002] Our company, as a producer of iron phosphate, has an annual production capacity of 40,000 tons of iron phosphate. During the operation of the iron phosphate plant, a large amount of tailings are generated. The tailings are small particulate powders carried in the calcination tail gas. Essentially, they are small particulate target products with high sulfur content. Since the sulfur content of the tailings is extremely high, exceeding 10,000 ppm, directly returning the tailings to the production system would cause the sulfur content of the products to exceed the standard. However, if the tailings cannot be returned to the system, it will cause problems such as loss of usable materials and high raw material consumption.

[0003] Therefore, our company initially conducted a trial operation by modifying the primary resizing tank of the tailings return washing system as a recycling point. The main problems encountered during the recycling trial operation were as follows: First, the washing system has a total of five stages, of which the first three stages are plate and frame filter presses, which operate intermittently, and the last two stages are centrifuge filters, which operate continuously. Since the washing steps of the previous and next stages in the plate and frame washing are one-to-one, adding material will disrupt the operating rhythm, causing the material in the primary resizing tank to accumulate more and more, affecting the normal operation of the plate and frame filter presses in the first three stages of plate and frame washing and filtration. Second, the discharge chute in the tailings resizing tank is prone to clogging when feeding material. Third, the mainstream production technology of ferric phosphate is the liquid phase precipitation method, which mainly uses ferric sulfate and phosphate as raw materials. The iron-phosphorus ratio is controlled to be <1 in the aqueous solution, phosphoric acid is added for aging, and then dried and calcined to produce ferric phosphate product. In the process of preparing ferric phosphate using this method, the product will go through two stages: crystal nucleation and crystal growth. In the initial stage of the reaction, a large number of fine iron phosphate crystal nuclei are formed, with small particle size and low sulfur content. In the nucleus growth stage, sulfate is wrapped around the nuclei during the growth process to form FePO4•xMSO4, which increases both particle size and sulfur content, affecting product quality and lithium iron phosphate battery performance.

[0004] Since the waste product generated by the iron phosphate system has already undergone calcination and high-temperature aging, directly adding water and returning it to the unaged new reaction material for recycling would affect the crystallization process during aging, resulting in a higher sulfur content and lower specific surface area in the product. Specific surface area is a particularly important physical property of battery materials, significantly impacting slurry formulation, electrode coating, initial coulombic efficiency, and cycle performance. A low specific surface area means the product specifications cannot meet market demands. Therefore, a new iron phosphate waste product recycling process is urgently needed to solve these problems. Summary of the Invention

[0005] The present invention provides a process for recycling tailings from the calcination of ferric phosphate, in order to solve the problems that the tailings after high-temperature aging are difficult to recover in the ferric phosphate system, and that the recovered product has a high sulfur content and a low specific surface area.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a process for recycling tailings powder from the calcination stage of ferric phosphate, comprising the following steps: S1: Add the Nth stage filtrate to the tailings reslurry tank to 80% level, and control the temperature of the filtrate within 50℃~70℃; the Nth stage filtrate is the filtrate obtained from the final stage of solid-liquid centrifugation in the system. S2: The tailings powder from the calcination section of ferric phosphate is transported to the tailings re-slurry tank. The mass ratio of the tailings powder to the filtrate from the calcination section of ferric phosphate is 1:15~1:25. The tailings re-slurry tank is circulated and re-slurried using a tailings re-slurry tank pump. The stirring time is ≥30min. S3: The well-stirred ferric phosphate calcination section tail powder slurry is transported to the aging buffer tank. The liquid level in the aging buffer tank is controlled at more than 50%. The mass of ferric phosphate calcination section tail powder added to the aging buffer tank does not exceed 5% of the original production line per ton of product. The temperature of the aging buffer tank is ≥90℃. S4: The mixed slurry in the aging buffer tank is fed into the N-1 stage centrifuge of the iron phosphate production system for solid-liquid separation. The separated iron phosphate solid residue is transported to the N-1 stage re-slurry tank of the iron phosphate production system for washing and pulping. The temperature of the washing and pulping water is ≥50℃. S5: Pass the mixed slurry after washing and pulping in stage N-1 into the Nth stage centrifuge for solid-liquid separation to obtain filter cake; S6: The filter cake is conveyed to the drying section to remove free water and obtain dried material. The drying temperature is controlled within 120℃~145℃. S7: The dried material is transported to the high-temperature calcination section. The calcination process includes the first, second, third and fourth temperature zones, with the temperatures of each zone controlled at 600±10℃, 650±10℃, 680±10℃ and 680±10℃ respectively. After calcination, the finished product iron phosphate is obtained.

[0007] The basic principle of this scheme is as follows: The product tailings generated by the ferric phosphate system are materials that have undergone calcination and high-temperature aging. If they are directly mixed with water and returned to the new, unaged reactants for recycling, the tailings with excessively high sulfur content will act as seed crystals and dispersants. Seed crystals act as starting points during crystallization, attracting solute molecules through their surface energy and active sites, promoting crystal formation and growth. Dispersants, on the other hand, can adsorb onto the surface of solid particles, thus affecting crystal transformation during material aging, resulting in high sulfur content and low specific surface area in the product. This process processes the ferric phosphate tailings in batches. After re-pulping one batch of tailings and completing one washing, the pulverized tailings slurry is divided into small portions and fed into the aging buffer. The material in the slurry is heated together with the normal material from the original production line, and the mass ratio between the original production material and the tailings is controlled. The tailings mass does not exceed 5% of the mass of each ton of original production material. This ensures that the original production material acts as the crystal nucleus for crystal transformation, while also adsorbing the target substances in the tailings for precipitation. Heating the tailings slurry can effectively improve the precipitation efficiency. The aged and buffered tailings slurry is then passed into the N-1 stage centrifugal solid-liquid separation of the original ferric phosphate production system. The resulting filter cake is passed into the N-1 stage re-slurry tank of the ferric phosphate production system and mixed with the product for another slurrying to complete the secondary washing. After the tailings and the product are slurried together to form a mixed slurry, it is then centrifuged in the Nth stage. Both are then dried and calcined together to obtain the target finished product of the ferric phosphate production system.

[0008] The beneficial effects of this plan are: 1. Laboratory experiments have verified that the sulfur content of the product obtained after tailings recovery meets market demand. This solution changes the recovery point of the tailings in the iron phosphate production system. Based on the fact that the product slurry in the N-1 stage centrifugation and re-slurrying process of the original production system has been aged, the tailings are recovered by adding no more than 5% tailings per ton of product after aging. This will not affect the operational balance of the first three stages of plate and frame washing, nor will it affect the product quality, and can effectively solve the problem of difficult tailings recovery.

[0009] 2. The tail powder is circulated and re-slurried in the tail powder re-slurry tank. That is, while the tail powder is being fed in, the tail slurry is circulated to the tail powder feeding point. The liquid fluidity of the slurry is used to avoid the problem of clogging of the feed chute.

[0010] 3. Taking a five-stage solid-liquid separation process as an example, the reslurry tailings are returned to the original production system in the fourth-stage centrifuge. The reslurry tailings are then re-slurried using the fifth-stage filtrate, which is the liquid phase separated from the fourth-stage reslurry, ensuring consistency. Furthermore, in the original production line's five-stage solid-liquid separation process, the first three stages are plate and frame filter presses, operating intermittently and corresponding one-to-one with the next stage of washing. Feeding would disrupt the plate and frame filter press's operating rhythm. In this solution, the fourth-stage reslurry tailings undergo solid-liquid separation using centrifuge filtration. The amount of slurry does not affect operation, and recovering the tailings at this point does not disrupt the original production line's continuous production process.

[0011] 4. Because the first-stage filtration pressure is high, and the remaining solid filter cake decreases with each stage of washing and filtration, the subsequent filtration pressure is not as high. Therefore, the later the recovery stage, the lower the recovery pressure. However, later recovery stages lead to a higher residual sulfur content. This solution, while delaying the recovery stage, also reduces residual sulfur by washing the product along with the original normal material, thus ensuring product quality.

[0012] Furthermore, the iron phosphate production system includes N-1 stages of washing and pulping, N stages of solid-liquid centrifugation, and N stages of filtrate obtained during the Nth stage of solid-liquid centrifugation.

[0013] Furthermore, the iron phosphate production system includes a total of 4 stages of washing and pulping and 5 stages of solid-liquid centrifugation separation; the mixed slurry in the aging buffer tank in S4 is centrifuged in the 4th stage to obtain iron phosphate solid residue, which is then transported to the 4th stage re-slurry tank for re-slurrying, and the mixed slurry after the 4th stage re-slurrying in S5 is fed into the 5th stage centrifuge.

[0014] Furthermore, the mass ratio of the tail powder to the filtrate in the calcination section of ferric phosphate in S2 is 1:20.

[0015] Furthermore, the mixing time in the tailings re-slurry tank of S2 is ≤6h.

[0016] Furthermore, the tailings reslurry tank is connected to a fifth-stage filtrate tank via a fifth-stage filtrate pump.

[0017] Furthermore, a discharge chute is connected to the top of the tailings re-slurry tank, and a tailings pump is connected to the upper end of the discharge chute via a pipe. The tailings pump is connected to the lower end of the tailings re-slurry tank via a pipe.

[0018] Furthermore, the upper end of the feeding chute is funnel-shaped.

[0019] Furthermore, the upper end of the aging buffer tank is connected to the end of the tail material pump that is connected to the discharge chute.

[0020] Furthermore, in S3, the amount of iron phosphate tailings slurry delivered to the aging buffer tank each time is 20% of the liquid level in the tailings re-slurry tank. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of an embodiment of the present invention. Detailed Implementation

[0022] The following detailed description illustrates the specific implementation method: The reference numerals in the accompanying drawings include: 1. Five-stage filtrate tank; 2. Five-stage filtrate pump; 3. Tail re-slurry tank; 31. Feed chute; 4. Tail pump; 5. Aging buffer tank.

[0023] The basic implementation examples are as follows: Figures 1 to 2 As shown: The production process of ferric phosphate includes raw material dissolution, reactor synthesis, aging, pressure filtration, slurry preparation, washing and pressure filtration, flash drying, and calcination. The slurry preparation and washing and pressure filtration steps involve four stages of washing and pulping, and five stages of solid-liquid separation to obtain the required ferric phosphate solid slag. The first three stages of the five-stage solid-liquid separation are performed by plate and frame filter presses, operating intermittently, with each stage's filtration rate corresponding to the next. The last two stages are performed by horizontal screw centrifuges, a continuous production process where the amount of slurry does not affect operation.

[0024] The structural schematic diagram of tailings recovery in this embodiment is shown below. Figure 2 As shown, it includes a five-stage filtrate tank 1, a five-stage filtrate pump 2, a tailings re-slurry tank 3, a tailings pump 4, and an aging buffer tank 5. The lower end of the five-stage filtrate tank 1 is connected to the upper part of the tailings re-slurry tank 3 through the five-stage filtrate pump 2, which is used to re-slurry the five-stage filtrate with the tailings. A vertical discharge chute 31 is installed at the top of the tailings re-slurry tank 3, and the upper end of the discharge chute 31 is a funnel shape with the opening facing upward.

[0025] The lower end of the tailings re-pulping tank 3 is connected to the upper end of the discharge chute 31 and the upper end of the aging buffer tank 5 via the tailings pump 4. Thus, the slurry in the tailings re-pulping tank 3 can be pumped from the tailings pump 4 into the discharge chute 31, and then enter the tailings re-pulping tank 3 along the discharge chute 31, completing the circulation pumping inside the tailings re-pulping tank 3. At the same time, the circulated tailings slurry can also be pumped into the next stage aging buffer tank 5 to complete the outward transportation.

[0026] A process for recycling tailings powder from the calcination of ferric phosphate includes the following steps: S1: Start the 5-stage filtrate transfer pump to pump the 5-stage filtrate from the 5-stage filtrate tank 1 into the tailings reslurry tank 3. The 5-stage filtrate level in the tailings reslurry tank 3 should be increased to 80% capacity, and the filtrate temperature should be 50℃~70℃. In this embodiment, the volume of the tailings reslurry tank at 80% capacity is 4m³. 3 .

[0027] S2: The tailings powder from the calcination section of ferric phosphate is transported to the tailings re-slurry tank 3. The mass ratio of tailings powder to filtrate from the calcination section of ferric phosphate is 1:20. During the feeding process, the tailings slurry from the calcination section of ferric phosphate is circulated and pumped in the tailings re-slurry tank 3 using the tailings pump 4. The tailings re-slurry tank 3 includes a stirring mechanism. The tailings slurry in the tailings re-slurry tank 3 can be used after stirring for more than 30 minutes. The stirring time of the tailings re-slurry tank 3 is ≤6 hours. S3: The well-stirred ferric phosphate calcination tail powder slurry is pumped to the aging buffer tank 5 via tail material pump 4. The temperature of the aging buffer tank 5 is controlled at ≥90℃, and the liquid level in the aging buffer tank 5 is controlled at ≥50% before receiving the tail material slurry. The mass of ferric phosphate calcination tail powder added to the aging buffer tank 5 does not exceed 5% of the mass of each ton of normal product. Furthermore, the tail material pump 4 adds the ferric phosphate calcination tail powder slurry to the aging buffer tank 5 in multiple batches, with each batch adding 20% ​​of the liquid level in the tail material slurry tank 3. In this embodiment, to prevent excessive addition of high-sulfur tail powder from affecting the desulfurization rate, the amount added each time is 1m³. 3 By increasing the temperature during the aging stage, high temperatures can effectively improve the sedimentation efficiency of the slurry.

[0028] S4: The mixed slurry in the aging buffer tank 5 is fed into the fourth-stage centrifuge in the iron phosphate production system for solid-liquid separation. The separated iron phosphate solid residue is transported to the fourth-stage re-slurry tank in the iron phosphate production system for re-slurrying. The temperature of the re-slurry water is controlled at ≥50℃. S5: The mixed slurry after the fourth stage of re-slurry is passed into the fifth stage centrifuge for solid-liquid separation to obtain filter cake.

[0029] S6: The filter cake is conveyed to the drying section to remove free water and obtain dried material. The drying temperature is controlled within 120℃~145℃.

[0030] S7: The dried material is transported to the high-temperature calcination section. The calcination process includes four temperature zones: 600±10℃, 650±10℃, 680±10℃, and 680±10℃, respectively.

[0031] Example 2 Unlike Example 1, in S2, the mass ratio of the tail powder from the calcination section of ferric phosphate to the filtrate is 1:15.

[0032] Example 3 Unlike Example 1, in S2, the mass ratio of the tail powder from the calcination section of ferric phosphate to the filtrate is 1:25.

[0033] Example 4 Unlike Example 1, the temperature of the re-slurry in S4 is 90°C.

[0034] Comparative Example 1 Unlike Example 1, S1 also includes an additive, namely 0.5% citric acid.

[0035] Comparative Example 2 Unlike Example 1, S1 also includes an additive, namely 1% citric acid.

[0036] Comparative Example 3 Unlike Example 1, S1 also includes an additive, namely 0.2 g / L of phosphoric acid.

[0037] Comparative Example 4 Unlike Example 1, S1 also includes an additive, namely 0.5 g / L of phosphoric acid.

[0038] Because the first-stage filtration pressure is high, and the remaining solid filter cake decreases with each stage of washing and filtration, the subsequent filtration pressure is not as high. Therefore, the later the recovery stage, the lower the recovery pressure. However, later recovery stages lead to fewer washing steps for sulfur-containing target products, resulting in a larger residual sulfur content. Therefore, this solution can reduce residual sulfur while delaying the recovery stage, as verified in the following experiments.

[0039] Statistical analysis of experiments in Examples 1-4 and Comparative Examples 1-4: (1) Experiment on the effect of washing temperature (re-slurry temperature) on the desulfurization effect in tailings powder

[0040] Based on the above data analysis, under the same conditions, adjusting the washing temperature does not significantly change the sulfur content of the final product, remaining within the error range. However, increasing the temperature does show a trend of decreasing sulfur content in the product. In contrast, the sulfur content in the dried sample does not exhibit a clear pattern of change. Therefore, in summary, washing temperature has some influence, but it is not a key parameter for desulfurization.

[0041] (2) Experiment on the effect of washing water volume on desulfurization effect in tailings powder

[0042] Based on the above data analysis, under the same conditions, adjusting the mass ratio, i.e., the amount of washing water, significantly changes the sulfur content in the final product, reducing it by more than 50 ppm. The sulfur content in the dried material also shows a decreasing trend with increasing washing water volume. Combined with the above analysis of washing temperature, the sulfur data for the experimental group (1:25, 90 degrees Celsius) shows some anomalies. Therefore, in conclusion, increasing the washing water volume can improve the desulfurization effect.

[0043] (3) Effect of washing times on desulfurization efficiency in tailings powder

[0044] Based on the above data analysis, under the same conditions, adjusting the number of washes—specifically, adding one more wash—significantly reduced the sulfur content in the dried samples. Due to differences in raw material samples, only one set of product data demonstrates that increasing the number of washes reduces the sulfur content in the product. The other sets did not show significant differences. However, the dried material data suggests that increasing the number of washes significantly reduces the sulfur content in the samples before calcination.

[0045] (4) Experiment on the effect of additives on the desulfurization effect in tailings powder

[0046] The method of using additives to alter the solubility in solution to address desulfurization is employed. Analysis of the above data shows that, under the same conditions, comparing the addition of different concentrations of citric acid to the solution with the absence of citric acid, there was no significant change in sulfur content in either the dried sample or the product, indicating a minor impact. All results were within the margin of error. Due to the addition of phosphoric acid, two washes were performed to ensure a normal iron-to-phosphorus ratio in the solution. Therefore, based on the dried sample data, adding phosphoric acid was more effective than adding citric acid. However, the addition of phosphoric acid did not show a significant change in the sulfur content of the product. In conclusion, the use of additives has little effect on desulfurization.

[0047] (5) Effect of calcination temperature on desulfurization effect and specific surface area in tailings powder

[0048] The sulfur content in the product varies significantly depending on the calcination temperature. High-temperature conditions can reduce the sulfur content to between 130-160 ppm, but the specific surface area also decreases accordingly, falling below 3.5 m² / g. Considering the current on-site calcination temperature of approximately 680℃, the sulfur content can only be reduced to around 130 ppm. Therefore, increasing the calcination temperature significantly improves desulfurization, but the specific surface area must also be considered.

[0049] The following conclusions can be drawn from the analysis of the experimental data: The main influencing factors for tailings desulfurization are washing water volume, washing frequency, and calcination temperature. In terms of influence and priority, the order is: calcination temperature > washing frequency > washing water volume.

[0050] Based on actual production conditions, the preferred tailings processing conditions for this process are: a washing-to-water ratio of 1:20, a washing time of 1 hour, and two washing cycles. The tailings are then mixed with the aged liquid from the production system and fed into the flash evaporation and calcination systems (680℃). By selecting the tailings recovery point at the fourth-stage resizing stage of the original production system, this process ensures that the fresh materials from the original system can complete their aging process. It also avoids excessive pressure on the plate and frame filtration process in the first-stage resizing stage caused by the tailings being resized before entering the first-stage resizing stage. Mixing the tailings with fresh materials ensures uniform calcination of the tailings, preventing sintering and ensuring no impact on the sulfur content of the product.

[0051] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A process for recycling tailings powder from the calcination of ferric phosphate, characterized in that: Includes the following steps: S1: Add the Nth stage filtrate to the tailings reslurry tank to 80% level, and control the temperature of the filtrate within 50℃~70℃; the Nth stage filtrate is the filtrate obtained from the final stage of solid-liquid centrifugation in the system; the Nth stage is 5 stages; S2: The tailings powder from the calcination section of ferric phosphate is transported to the tailings re-slurry tank. The mass ratio of the tailings powder to the filtrate from the calcination section of ferric phosphate is 1:15~1:

25. The tailings re-slurry tank is circulated and re-slurried using a tailings re-slurry tank pump. The stirring time is ≥30min. S3: The well-stirred ferric phosphate calcination section tail powder slurry is transported to the aging buffer tank. The liquid level in the aging buffer tank is controlled at more than 50%. The mass of ferric phosphate calcination section tail powder added to the aging buffer tank shall not exceed 5% of the product per ton. The temperature of the aging buffer tank shall be ≥90℃. S4: The mixed slurry in the aging buffer tank is fed into the N-1 stage centrifuge of the iron phosphate production system for solid-liquid separation. The separated iron phosphate solid residue is transported to the N-1 stage re-slurry tank of the iron phosphate production system for washing and pulping. The temperature of the washing and pulping water is ≥50℃. S5: Pass the mixed slurry after washing and pulping in stage N-1 into the Nth stage centrifuge for solid-liquid separation to obtain filter cake and Nth stage filtrate; S6: The filter cake is conveyed to the drying section to remove free water and obtain dried material. The drying temperature is controlled within 120℃~145℃. S7: The dried material is transported to the high-temperature calcination section. The calcination process includes the first, second, third and fourth temperature zones, with the temperatures of each zone controlled at 600±10℃, 650±10℃, 680±10℃ and 680±10℃ respectively. After calcination, the finished product iron phosphate is obtained.

2. The process for recycling tailings powder from the calcination stage of ferric phosphate as described in claim 1, characterized in that: The mixed slurry in the aging buffer tank in S4 is centrifuged in the fourth stage to obtain iron phosphate solid residue. The iron phosphate solid residue is transported to the fourth stage re-slurry tank for re-slurrying. The mixed slurry after the fourth stage re-slurrying in S5 is fed into the fifth stage centrifuge.

3. The process for recycling tailings powder from the calcination section of ferric phosphate as described in claim 2, characterized in that: The mass ratio of the tailings powder to the filtrate in the calcination section of ferric phosphate in S2 is 1:

20.

4. The process for recycling tailings powder from the calcination stage of ferric phosphate as described in claim 1, characterized in that: The mixing time in the tailings re-slurry tank of S2 is ≤6h.

5. The process for recycling tailings powder from the calcination section of ferric phosphate as described in claim 2, characterized in that: The tailings reslurry tank is connected to a fifth-stage filtrate tank via a fifth-stage filtrate pump.

6. The process for recycling tailings powder from the calcination section of ferric phosphate as described in claim 5, characterized in that: The top of the tailings re-slurry tank is connected to a discharge chute, and the upper end of the discharge chute is connected to a tailings pump through a pipe. The tailings pump is connected to the lower end of the tailings re-slurry tank through a pipe.

7. The process for recycling tailings powder from the calcination section of ferric phosphate as described in claim 6, characterized in that: The upper end of the feeding chute is funnel-shaped.

8. The process for recycling tailings powder from the calcination section of ferric phosphate as described in claim 7, characterized in that: The upper end of the aging buffer tank is connected to the end of the tail material pump that is connected to the discharge chute.

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