A method for preparing high-purity trisodium phosphate by taking phosphorus iron slag as a phosphorus source

Abstract

This invention provides a method for preparing high-purity trisodium phosphate from iron phosphate slag as a phosphorus source, including defluorination and solid fluoride roasting of raw materials; enhanced low-temperature alkaline leaching with composite flux; high-temperature solid-liquid separation and countercurrent washing; synergistic purification based on complexation and nanofiltration; evaporation concentration, cooling crystallization, and mother liquor or condensate circulation; enhanced low-temperature alkaline leaching with composite flux and utilizing the synergistic impurity removal mechanism of boron complexation-nanofiltration membrane to achieve efficient resource utilization of iron phosphate slag, solving the problems of high energy consumption, complex process, high product impurities, and low phosphorus yield in existing technologies.

Description

Technical Field

[0001] This invention relates to the fields of industrial solid waste resource utilization and phosphorus chemical technology, and in particular to a method for preparing high-purity trisodium phosphate using phosphorus iron slag as a phosphorus source. Background Technology

[0002] With the rapid development of the new energy vehicle industry, the demand for lithium iron phosphate (LFP) batteries has increased significantly, leading to a year-on-year rise in the amount of waste LFP batteries generated annually. The process of producing lithium carbonate from recycled LFP battery materials produces a large amount of iron phosphate slag, whose main component is iron phosphate (91-93% by mass), along with carbon (6-8%) and small amounts of impurity ions such as aluminum, silicon, calcium, and fluorine. Currently, this iron phosphate slag is mostly disposed of through stockpiling, which not only occupies land resources but may also cause soil and water pollution, while wasting valuable phosphorus resources.

[0003] Existing technologies include the recovery of phosphorus resources from ferrophosphate slag. For example, trisodium phosphate is prepared by alkaline leaching of ferrophosphate slag with sodium hydroxide, followed by purification with cation exchange resin to obtain a relatively pure product. Other methods employ oxidative roasting followed by flotation to recover ferric phosphate and graphite, but these suffer from problems such as long process flows, limited ferrophosphate recovery rates, and high acid and alkali consumption. Furthermore, traditional trisodium phosphate production relies heavily on phosphate rock, involving complex processes such as sulfuric acid leaching, which faces challenges related to phosphate rock resource shortages and significant environmental pressures.

[0004] Existing technologies for preparing trisodium phosphate from ferrophosphate slag have several shortcomings: First, the alkaline leaching reaction temperature is high (usually above 90℃), resulting in high energy consumption; second, the purification process uses cation exchange resin, leading to frequent resin regeneration, high production costs, and incomplete impurity removal; third, the process often involves subsequent steps such as acidification and freezing, making the process complex and increasing energy consumption and costs. Therefore, there is an urgent need to develop a simple, low-energy-consumption method for preparing trisodium phosphate from ferrophosphate slag that achieves good impurity removal and high product purity. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing a method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source, thereby solving the problems of high energy consumption, complex processes, high product impurities, and low phosphorus yield in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source, comprising the following steps: S1. Defluorination and solidification roasting of raw materials; S2, composite flux enhances low-temperature alkaline immersion; S3, High-temperature solid-liquid separation and countercurrent washing; S4. Synergistic purification based on complexation and nanofiltration; S5. Evaporation and concentration, cooling and crystallization, and circulation of mother liquor or condensate; S6. Iron resource recycling.

[0007] Furthermore, S1 includes: S101. Take the phosphorus iron slag after lithium extraction from lithium iron phosphate battery recycled material as raw material, crush and screen the phosphorus iron slag, and control the particle size after screening to ≤100 mesh to obtain pre-treated slag with uniform particle size. S102. Add 0.5-2% of the pretreated slag mass of the fluorine-fixing agent to the pretreated slag, place the mixture in a roasting furnace, and roast it in an oxygen atmosphere at 300-400℃ for 1.5-2.5 hours.

[0008] Furthermore, S2 specifically refers to: S201. Slurry preparation: Add the calcined residue to the reactor and add pure water or process recycled water at a solid-liquid mass-volume ratio of 1:4 to 1:1.2 to prepare the slurry. S202. Under stirring conditions, add sodium hydroxide to the slurry at a molar ratio of 1.02 to 1.08 times the theoretical amount; at the same time, add 1 to 5% of the mass of the calcined slag as a composite flux. S203, Low-temperature leaching reaction: Control the reaction temperature at 60~75℃, the stirring speed at 300~500 rpm, and the reaction time at 1.5~2.5 hours to obtain alkali-leached slurry.

[0009] Furthermore, S3 specifically refers to: S301. Hot filtration: The alkaline leaching slurry after the reaction is completed is filtered while hot at a temperature of 65~70℃ to obtain a clear trisodium phosphate crude liquor and an alkaline leaching residue whose main component is ferric hydroxide. S302, Countercurrent Washing: The obtained alkaline leaching residue is subjected to three countercurrent washing processes, specifically: The washing liquid of the next stage is used on the washing residue of the previous stage, while the third wash uses pure water or evaporated condensate. Finally, the filtration process yields a high-concentration washing liquid and pure ferric hydroxide filter residue. All of the washing liquid is returned to S201 to replace part of the pure water for pulping the roasted residue.

[0010] Furthermore, S4 specifically includes: S401, Nanofiltration separation: The crude trisodium phosphate solution obtained in S3 is passed through a two-stage series nanofiltration membrane separation system. The nanofiltration membrane separation system operates in a cross-flow filtration mode to reduce membrane fouling. The operating pressure is controlled at 0.4~0.6MPa, the temperature at 40~50℃, and the concentration factor is controlled at 3~5 times. S402, Synergistic Impurity Removal Mechanism: A nanofiltration membrane with a molecular weight cutoff of 100~150 Da is selected. At this cutoff precision, phosphate and sodium ions with smaller hydration diameters can pass through the membrane pores smoothly; while the aluminum and silicon macromolecular complexes formed by borax complexation in S2 are efficiently retained because their size is larger than the membrane pore size. S403. Product and Waste Treatment: High-purity trisodium phosphate purified solution is obtained from the permeate side; the content of each impurity ion is reduced to 10 mg / L or less. The impurity concentrate obtained from the retrieval side contains aluminum, silicon, and calcium complexes and excess alkali. After neutralization treatment with lime slurry, it meets the discharge standards.

[0011] Furthermore, S5 specifically includes: S501, Evaporation and Concentration: The purified trisodium phosphate solution is fed into an evaporator crystallizer and evaporated and concentrated under negative pressure conditions of 0.06~0.08MPa and 80~90℃. When the concentration of trisodium phosphate in the solution reaches 600~650g / L, evaporation is stopped. The large amount of condensate generated during the evaporation process is collected and reused for washing the alkaline leaching residue in S3. S502, Cooling Crystallization and Drying: Slowly cool the concentrated solution to 20~30℃ to allow trisodium phosphate to crystallize out as dodecahydrate; then separate the trisodium phosphate crystals and the mother liquor by centrifugation; dry the separated trisodium phosphate crystals at 200~250℃ for 2~3h to remove the water of crystallization, and finally obtain anhydrous high-purity trisodium phosphate with a purity ≥98.5%.

[0012] Furthermore, S6 specifically includes: The ferric hydroxide filter residue obtained from the S3 solid-liquid separation is added to a calcining furnace and calcined at 600~700℃ under air conditions for 2~3 hours. During this process, the ferric hydroxide decomposes under heat and is eventually converted into ferric oxide.

[0013] Furthermore, the fluorine-fixing agent is one of calcium carbonate, calcium oxide, or calcium hydroxide.

[0014] Furthermore, the theoretical quantity refers to the stoichiometric calculation based on the complete reaction of ferric phosphate in ferric phosphate slag to produce trisodium phosphate and ferric hydroxide; The composite flux is composed of sodium carbonate and borax mixed in a mass ratio of 3:1.

[0015] The beneficial effects of this invention are as follows: by strengthening low-temperature alkaline leaching with composite flux and utilizing the boron complex-nanofiltration membrane synergistic impurity removal mechanism, the resource-efficient utilization of phosphorus iron slag is achieved, solving the problems of high energy consumption, complex process, high product impurities and low phosphorus yield in the existing technology. By adding sodium carbonate-borax composite flux, efficient leaching of phosphorus iron slag can be achieved at a low temperature of 60~75℃. Compared with the traditional high-temperature alkaline leaching process above 90℃, the overall energy consumption is reduced by about 40%. Moreover, the pretreatment roasting temperature is only 300~400℃, which further reduces energy consumption. By utilizing the property of borax to form macromolecular complexes with silica and aluminate, and combined with the sieving effect of a 100~150Da nanofiltration membrane, the impurity removal rate reaches over 99%, and the impurity content of the purified liquid is ≤10mg / L. Through a recycling system that combines countercurrent washing and evaporation mother liquor recycling, the comprehensive recovery rate of phosphorus reaches over 98.5%, and the comprehensive recovery rate of iron reaches over 97%, achieving full-component resource utilization of solid waste. The final anhydrous trisodium phosphate product has a purity of ≥98.5% and can be widely used in high-end fields such as water treatment; the by-product ferric oxide has a purity of ≥97% and can be used in the coatings and ceramics industries, with high economic added value. Attached Figure Description

[0016] Figure 1 This is a flowchart of a method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0018] Please see Figure 1 A method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source includes the following steps: S1. Defluorination and solidification roasting of raw materials; S2, composite flux enhances low-temperature alkaline immersion; S3, High-temperature solid-liquid separation and countercurrent washing; S4. Synergistic purification based on complexation and nanofiltration; S5. Evaporation and concentration, cooling and crystallization, and circulation of mother liquor or condensate; S6. Iron resource recycling.

[0019] S1 includes: S101. Take the phosphorus iron slag after lithium extraction from lithium iron phosphate battery recycled material as raw material, crush and screen the phosphorus iron slag, and control the particle size after screening to ≤100 mesh to obtain pre-treated slag with uniform particle size; so as to improve the contact area and efficiency of subsequent reactions.

[0020] Phosphorus iron slag is the waste residue generated during the lithium extraction process of lithium iron phosphate batteries. Its main components, by mass fraction, are: 91-93% iron phosphate, 6-8% carbon, and the remainder being aluminum, silicon, calcium, and fluorine impurities. S102. Add 0.5-2% of the pretreated slag mass of the fluorine-fixing agent to the pretreated slag, place the mixture in a roasting furnace, and roast it in an aerobic atmosphere at 300-400℃ for 1.5-2.5 hours.

[0021] The purpose of this process is to remove carbon powder and organic binders from the iron phosphate slag through aerobic roasting, so as to avoid interfering with subsequent leaching and product purity; in addition, the fluoride-fixing agent reacts with the fluorides in the slag to generate stable calcium fluoride precipitate, which fixes the fluorine in the roasted slag and prevents it from entering the subsequent liquid phase and causing equipment corrosion and product contamination. After roasting, the slag is obtained by natural cooling.

[0022] Specifically, S2 is: S201. Slurry preparation: Add the calcined residue to the reactor and add pure water or process recycled water at a solid-liquid mass-volume ratio of 1:4~1:1.2 kg / L to prepare the slurry. S202. Under stirring conditions, add sodium hydroxide to the slurry at a molar ratio of 1.02 to 1.08 times the theoretical amount; at the same time, add 1 to 5% of the mass of the calcined slag as a composite flux. The theoretical quantity is calculated based on the equation FePO4 + NaOH = Fe(OH)3 + Na3PO4.

[0023] S203, Low-temperature leaching reaction: Control the reaction temperature at 60~75℃, the stirring speed at 300~500 rpm, and the reaction time at 1.5~2.5 hours to obtain alkali-leached slurry.

[0024] The key to this step lies in the synergistic effect of the composite flux: borax dissociates in an alkaline solution to generate [B(OH)4]. - The active boron group can be inserted into the interstitial spaces of the iron phosphate lattice, weakening the Fe... 3+ With PO4 3- The bond energy between them; sodium carbonate can adjust the ionic strength of the system, reduce the viscosity of the slurry, and accelerate the OH- reaction. - Na + It diffuses onto the surface of ferric phosphate particles; the synergistic effect of both reduces the alkaline leaching temperature from over 90℃ to below 75℃, significantly lowering energy consumption. Simultaneously, the borate ions (B(OH)4) generated by the hydrolysis of borax under alkaline conditions... - ), can react with leached aluminum, silicon and other impurity acid anions (such as Al(OH)4) - SiO3 2- A complexation reaction occurs, forming anionic complexes with larger particle sizes, which creates favorable conditions for subsequent membrane separation and impurity removal.

[0025] Specifically, S3 is: S301. Hot filtration: The alkaline leaching slurry after the reaction is completed is filtered while hot at a temperature of 65~70℃ to obtain a clear crude trisodium phosphate solution and an alkaline leaching residue whose main component is ferric hydroxide. Hot filtration can prevent trisodium phosphate from crystallizing and precipitating at low temperatures and clogging filters or pipelines, thereby improving filtration efficiency.

[0026] S302, Countercurrent Washing: The obtained alkaline leaching residue is subjected to three countercurrent washing processes, specifically: The washing liquid from the next stage is used on the washing residue from the previous stage, while the third wash uses pure water or evaporated condensate; through countercurrent washing, the residual trisodium phosphate solution adsorbed on the surface of the alkaline leaching residue can be effectively recovered.

[0027] Countercurrent washing can effectively recover residual trisodium phosphate solution adsorbed on the surface of alkaline leaching residue.

[0028] Finally, the filtration process yields a high-concentration washing liquid and pure ferric hydroxide filter residue. All of the washing liquid is returned to step S201 to replace part of the pure water for pulping the roasted slag, thus achieving preliminary recovery of phosphorus resources and recycling of water.

[0029] Specifically, S4 is: S401, Nanofiltration separation: The crude trisodium phosphate solution obtained in S3 is passed through a two-stage series nanofiltration membrane separation system. The nanofiltration membrane separation system operates in a cross-flow filtration mode to reduce membrane fouling. The operating pressure is controlled at 0.4~0.6MPa, the temperature at 40~50℃, and the concentration factor is controlled at 3~5 times. S402, Synergistic Impurity Removal Mechanism: A nanofiltration membrane with a molecular weight cutoff of 100~150 Da is selected. At this cutoff precision, phosphate and sodium ions with smaller hydration diameters can pass smoothly through the membrane pores. Meanwhile, the aluminum and silicon macromolecular complexes formed by borax complexation in S2 are efficiently retained because their size is larger than the membrane pore size. This synergistic mechanism of "boron complexation pre-amplification - precise nanofiltration membrane retention" achieves deep removal of various impurity ions, with an impurity removal rate of over 99%.

[0030] S403. Product and Waste Treatment: High-purity trisodium phosphate purified solution is obtained from the permeate side; the content of each impurity ion is reduced to 10 mg / L or below. The impurity concentrate obtained from the interception side contains aluminum, silicon, and calcium complexes and excess alkali. After neutralization treatment with lime slurry, it meets the discharge standards.

[0031] Specifically, S5 is: S501, Evaporation and Concentration: The purified trisodium phosphate solution is fed into an evaporator crystallizer and evaporated and concentrated under negative pressure conditions of 0.06~0.08MPa vacuum and 80~90℃. When the concentration of trisodium phosphate in the solution reaches 600~650g / L, evaporation is stopped. The large amount of condensate generated during the evaporation process is collected and reused for washing the alkaline leaching residue in S3; thus achieving efficient recycling of water resources.

[0032] S502, Cooling Crystallization and Drying: Slowly cool the concentrated solution to 20~30℃ to allow trisodium phosphate to crystallize out as dodecahydrate; then separate the trisodium phosphate crystals and the mother liquor by centrifugation; dry the separated trisodium phosphate crystals at 200~250℃ for 2~3h to remove the water of crystallization, and finally obtain anhydrous high-purity trisodium phosphate with a purity ≥98.5%.

[0033] The mother liquor from the evaporation mainly contains uncrystallized trisodium phosphate, and it is all refluxed to the evaporator crystallizer for the next round of concentration.

[0034] Specifically, S6 is: The ferric hydroxide filter residue obtained from the S3 solid-liquid separation is added to a calcining furnace and calcined at 600~700℃ under air conditions for 2~3 hours. During this process, the ferric hydroxide decomposes under heat and is eventually converted into ferric oxide.

[0035] The fluorine-fixing agent is one of calcium carbonate, calcium oxide, or calcium hydroxide.

[0036] The theoretical quantity refers to the stoichiometric calculation based on the complete reaction of ferric phosphate in ferric phosphate slag to produce trisodium phosphate and ferric hydroxide. The composite flux is composed of sodium carbonate and borax mixed in a mass ratio of 3:1.

[0037] Example 1: S1. Defluorination and solidification roasting of raw materials: Take 100 kg of phosphorus-iron slag from lithium extraction of lithium iron phosphate battery recycled material. Its main components are: 91.5% iron phosphate and 7.1% carbon. Crush it through a 100-mesh standard sieve to obtain pretreated slag. Add 1.5 kg (i.e., 1.5% of the slag mass) of calcium oxide (CaO) to the pretreated slag as a defluorinating agent, and mix it evenly using a mixer; The mixture was placed in a calcining furnace and calcined at 400°C under aerobic conditions for 2 hours to remove carbon powder and organic binder and achieve fluorine fixation. After cooling, the calcined residue was obtained. S2, composite flux-enhanced low-temperature alkaline immersion: Add the above 50kg of roasted slag (98.34% iron phosphate, 0.21% carbon) to the reactor, add 500L of pure water (solid-liquid ratio 1:10), and start stirring to make slurry; Add 40.5 kg of sodium hydroxide (1.05 times the theoretical amount) and 1.5 kg of composite flux (a mixture of 1.125 kg of sodium carbonate and 0.375 kg of borax). The reaction temperature was controlled at 70±2℃, the stirring speed at 400r / min, and the reaction was carried out at a constant temperature for 2h to obtain the alkaline slurry. S3. High-temperature solid-liquid separation and countercurrent washing: After the reaction, the alkaline slurry was fed into a plate and frame filter press at 68°C while still hot to obtain a clear trisodium phosphate crude solution and ferric hydroxide filter residue. The ferric hydroxide filter residue was subjected to three countercurrent washings. The first wash used pure water, subsequent washes used the next stage washing liquid, and the final wash used the condensate generated in S5. This resulted in a high-concentration washing liquid and pure ferric hydroxide filter residue. All washing liquid was returned to S2 for use in the pulping of the next batch of roasted slag. S4. Synergistic purification based on complexation and nanofiltration: The crude trisodium phosphate solution was pumped into a nanofiltration membrane separation system. The system employed cross-flow filtration and used a nanofiltration membrane with a molecular weight cutoff of 150 Da. Control the operating pressure to 0.5 MPa and the temperature to 45°C. During the filtration process, control the concentration ratio to 4 times. The trisodium phosphate purified solution was obtained through nanofiltration membrane; the main impurity ion contents in the purified solution were as follows, as determined by ICP-MS: aluminum (Al) 6.4 mg / L, silicon (Si) 3.6 mg / L, calcium (Ca) 8.8 mg / L, fluorine (F) 5.6 mg / L, and iron (Fe) 3.7 mg / L. Retained liquid that did not pass through the nanofiltration membrane: volume 30L, phosphorus (P): 3.8g / L; S5. Evaporation, concentration, cooling, crystallization, and recycling: The purified trisodium phosphate solution was fed into an evaporator crystallizer and evaporated at a vacuum of 0.07 MPa and a temperature of 90°C until the trisodium phosphate was concentrated to a concentration of 600 g / L. The condensate was collected and used for washing the filter residue in step 3. The concentrate was cooled to 25°C and centrifuged to obtain trisodium phosphate dodecahydrate crystals. All the mother liquor was refluxed back to the evaporator crystallizer for the next round of concentration. The obtained trisodium dodecahydrate crystals were dried at 220℃ for 2.5h to remove the water of crystallization, yielding 42.70kg of anhydrous high-purity trisodium phosphate product. S6. Iron Resource Recycling: The ferric hydroxide filter residue obtained from S3 was fed into a roasting furnace and roasted at 650°C with excess air for 2.5 hours to completely decompose the ferric hydroxide, yielding 26.02 kg of ferric oxide.

[0038] Testing showed that the trisodium phosphate product obtained in this embodiment had a purity of 98.61% and a phosphorus recovery rate of 98.6%; the ferric oxide product had a purity of 97.34%, contained 0.11% phosphorus, and had an iron recovery rate of 97.2%. The overall energy consumption of the alkali leaching process was reduced by about 44% compared with the traditional high-temperature process.

[0039] Phosphorus recovery rate = 1 - (mass of phosphorus in retentate + mass of phosphorus in ferric oxide) / mass of phosphorus in 50kg roasting residue.

[0040] Iron recovery rate = mass of iron in ferric oxide product / mass of iron in 50kg roasting slag.

[0041] Example 2: S1. Defluorination and solidification roasting of raw materials: Take 100 kg of phosphorus-iron slag from other batches of lithium iron phosphate battery recycling materials after lithium extraction. Its main components are: 92.3% iron phosphate and 6.4% carbon. Crush it through a 100-mesh standard sieve to obtain pretreated slag.

[0042] Add 1.5 kg (i.e., 1.5% of the slag mass) of calcium oxide (CaO) and mix thoroughly.

[0043] The residue was calcined at 400℃ under aerobic conditions for 2 hours and then cooled to obtain calcined residue.

[0044] S2, composite flux-enhanced low-temperature alkaline immersion: Take 50 kg of roasted residue (98.64% iron phosphate, 0.25% carbon) and add 400 L of pure water (solid-liquid ratio 1:8) to make a slurry.

[0045] In this example, 38.88 kg of sodium hydroxide was added (1.02 times the theoretical amount).

[0046] Add 2 kg of composite flux (1.5 kg sodium carbonate + 0.5 kg borax), control the reaction temperature at 60 ± 2℃, stir at 300 rpm, and react for 2.5 h to obtain alkali-impregnated slurry; S3. High-temperature solid-liquid separation and countercurrent washing: The alkaline slurry was filtered while hot at 65°C to obtain crude trisodium phosphate solution and ferric hydroxide filter residue. The filter residue is washed three times in countercurrent to obtain washing liquid and pure ferric hydroxide filter residue. The washing liquid is returned to S2 pulping. S4. Synergistic purification based on complexation and nanofiltration: The crude trisodium phosphate solution was passed into a nanofiltration membrane system (molecular weight cutoff 150 Da) and cross-flow filtration was performed. Control the operating pressure to 0.4 MPa, temperature to 40℃, and concentration ratio to 3 times; A trisodium phosphate purified solution was obtained. The concentrations of impurity ions in the purified solution were measured as follows: aluminum 5.8 mg / L, silicon 4.2 mg / L, calcium 5.7 mg / L, fluorine 5.1 mg / L, and iron 2.8 mg / L. Retained liquid that did not pass through the nanofiltration membrane: volume 53 L, phosphorus (P): 2.2 g / L; S5. Evaporation and concentration, cooling and crystallization, and circulation of mother liquor or condensate: The purified solution was evaporated and concentrated to an anhydrous trisodium phosphate concentration of 630 g / L under a vacuum of 0.06 MPa and a temperature of 85 °C. The concentrate was cooled to 20°C to crystallize, and centrifuged to obtain trisodium phosphate crystals and evaporation mother liquor. The mother liquor was returned to the evaporator crystallizer. The obtained trisodium dodecahydrate crystals were dried at 220℃ for 2.5h to remove the water of crystallization, yielding 44.47kg of anhydrous high-purity trisodium phosphate product. S6. Iron Resource Recycling: Ferric hydroxide filter residue was calcined at 600℃ under air circulation for 3 hours to obtain 26.11 kg of ferric oxide; Testing showed that the trisodium phosphate product obtained in this embodiment had a purity of 98.66% and a phosphorus recovery rate of 98.53%; the ferric oxide product had a purity of 97.58%, contained 0.12% phosphorus, and had an iron recovery rate of 97.56%. The overall energy consumption of the alkali leaching process was reduced by approximately 42% compared to the traditional high-temperature process.

[0047] Phosphorus recovery rate = 1 - (mass of phosphorus in retentate + mass of phosphorus in ferric oxide) / mass of phosphorus in 50kg roasting residue.

[0048] Iron recovery rate = mass of iron in ferric oxide product / mass of iron in 50kg roasting slag.

[0049] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be defined by the appended claims.

Claims

1. A method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source, characterized in that, Includes the following steps: S1. Defluorination and solidification roasting of raw materials; S2, composite flux enhances low-temperature alkaline immersion; S3, High-temperature solid-liquid separation and countercurrent washing; S4. Synergistic purification based on complexation and nanofiltration; S5. Evaporation and concentration, cooling and crystallization, and circulation of mother liquor or condensate; S6. Iron resource recycling.

2. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 1, characterized in that, S1 includes: S101. Take the phosphorus iron slag after lithium extraction from lithium iron phosphate battery recycled material as raw material, crush and screen the phosphorus iron slag, and control the particle size after screening to ≤100 mesh to obtain pre-treated slag with uniform particle size. S102. Add 0.5-2% of the pretreated slag mass of the fluorine-fixing agent to the pretreated slag, place the mixture in a roasting furnace, and roast it in an oxygen atmosphere at 300-400℃ for 1.5-2.5 hours.

3. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 2, characterized in that, Specifically, S2 is: S201. Slurry preparation: Add the calcined residue to the reactor and add pure water or process recycled water at a solid-liquid mass-volume ratio of 1:4 to 1:1.2 to prepare the slurry. S202. Under stirring conditions, add sodium hydroxide to the slurry at a molar ratio of 1.02 to 1.08 times the theoretical amount; at the same time, add 1 to 5% of the mass of the calcined slag as a composite flux. S203, Low-temperature leaching reaction: Control the reaction temperature at 60~75℃, the stirring speed at 300~500 rpm, and the reaction time at 1.5~2.5 hours to obtain alkali-leached slurry.

4. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 3, characterized in that, Specifically, S3 is: S301. Hot filtration: The alkaline leaching slurry after the reaction is completed is filtered while hot at a temperature of 65~70℃ to obtain a clear trisodium phosphate crude liquor and an alkaline leaching residue whose main component is ferric hydroxide. S302, Countercurrent Washing: The obtained alkaline leaching residue is subjected to three countercurrent washing processes, specifically: The washing liquid of the next stage is used on the washing residue of the previous stage, while the third wash uses pure water or evaporated condensate. Finally, the filtration process yields a high-concentration washing liquid and pure ferric hydroxide filter residue. All of the washing liquid is returned to S201 to replace part of the pure water for pulping the roasted residue.

5. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 4, characterized in that, Specifically, S4 is: S401, Nanofiltration separation: The crude trisodium phosphate solution obtained in S3 is passed through a two-stage series nanofiltration membrane separation system. The nanofiltration membrane separation system operates in a cross-flow filtration mode to reduce membrane fouling. The operating pressure is controlled at 0.4~0.6MPa, the temperature at 40~50℃, and the concentration factor is controlled at 3~5 times. S402, Synergistic Impurity Removal Mechanism: A nanofiltration membrane with a molecular weight cutoff of 100~150 Da is selected. At this cutoff precision, phosphate and sodium ions with smaller hydration diameters can pass through the membrane pores smoothly; while the aluminum and silicon macromolecular complexes formed by borax complexation in S2 are efficiently retained because their size is larger than the membrane pore size. S403. Product and Waste Treatment: High-purity trisodium phosphate purified solution is obtained from the permeate side; the content of each impurity ion is reduced to 10 mg / L or below. The impurity concentrate obtained from the interception side contains aluminum, silicon, and calcium complexes and excess alkali. After neutralization treatment with lime slurry, it meets the discharge standards.

6. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 5, characterized in that, Specifically, S5 is: S501, Evaporation and Concentration: The purified trisodium phosphate solution is fed into an evaporator crystallizer and evaporated and concentrated under negative pressure conditions of 0.06~0.08MPa and 80~90℃. When the concentration of trisodium phosphate in the solution reaches 600~650g / L, evaporation is stopped. The large amount of condensate generated during the evaporation process is collected and reused for washing the alkaline leaching residue in S3. S502, Cooling Crystallization and Drying: Slowly cool the concentrated solution to 20~30℃ to allow trisodium phosphate to crystallize out as dodecahydrate; then separate the trisodium phosphate crystals and the mother liquor by centrifugation; dry the separated trisodium phosphate crystals at 200~250℃ for 2~3h to remove the water of crystallization, and finally obtain anhydrous high-purity trisodium phosphate with a purity ≥98.5%.

7. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 6, characterized in that, Specifically, S6 is: The ferric hydroxide filter residue obtained from the S3 solid-liquid separation is added to a calcining furnace and calcined at 600~700℃ under air conditions for 2~3 hours. During this process, the ferric hydroxide decomposes under heat and is eventually converted into ferric oxide.

8. The method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 2, characterized in that: The fluorine-fixing agent is one of calcium carbonate, calcium oxide, or calcium hydroxide.

9. A method for preparing high-purity trisodium phosphate using ferrophosphate slag as a phosphorus source according to claim 3, characterized in that: The theoretical quantity refers to the stoichiometric calculation based on the complete reaction of ferric phosphate in ferric phosphate slag to produce trisodium phosphate and ferric hydroxide. The composite flux is composed of sodium carbonate and borax mixed in a mass ratio of 3:1.

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