Method for recovering lithium from iron phosphate washing water synthesized by recovering all components of lithium iron phosphate black powder

Abstract

The invention discloses a method for recovering lithium in synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder, which comprises the following steps of: adding a sodium-containing alkaline substance into the synthetic washing water, and sequentially standing and separating to obtain impurity-removed liquid and impurity-removed slag; performing membrane concentration on the impurity-removed liquid to obtain concentrated water and fresh water; the concentrated water is sequentially subjected to extraction, carbon dioxide reverse extraction lithium precipitation, filtration and the like, and raffinate, lithium salt and lithium precipitation liquid are obtained. And returning the lithium precipitation liquid to the previous extraction process, adjusting the pH value of the raffinate, and carrying out evaporative crystallization to obtain a sodium sulfate crystal. The invention relates to the technical field of lithium recovery from iron phosphate wastewater. According to the method for recovering the lithium in the iron phosphate washing water by recovering and synthesizing all components of the lithium iron phosphate black powder, the iron phosphate is firstly extracted in the lithium iron phosphate all-component recovery, so that the economical efficiency and the risk resistance are higher, the recovery rate of the lithium in the washing water extracted and recovered is higher, and the lithium salt is purer. Fresh water obtained through membrane concentration reaches the pure water reuse standard and can be reused in iron phosphate production.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium recovery from iron phosphate wastewater, and specifically to a method for recovering lithium from the washing water of synthesized iron phosphate by fully recovering all components of lithium iron phosphate black powder. Background Art

[0002] Lithium iron phosphate batteries are widely used in electric bicycles, electric vehicles, electric buses and other fields due to their low price and high safety. With the rapid development of the new energy industry, the scrapping volume of lithium iron phosphate batteries has increased year by year. Therefore, recycling waste lithium iron phosphate batteries as raw materials for lithium salts and iron phosphate is a major trend nowadays. At present, the recovery of lithium iron phosphate mainly adopts the process of preferentially extracting lithium and then synthesizing iron phosphate. However, our company adopts the full-component recovery of lithium iron phosphate black powder, first synthesizes iron phosphate, and in the production process, it will go through processes such as synthesis and washing. The washing water generated contains not only sodium ions, phosphate ions, sulfate ions and other metal ions, but also a small amount of lithium ions. Lithium salts are indispensable raw materials in the production of lithium batteries and are also very rare resources. Therefore, it is very necessary to recover lithium from the synthesized washing water.

[0003] Currently, the mainstream methods for treating the washing water in the production of iron phosphate are lime precipitation method, magnesium salt treatment method and membrane treatment method. These methods only involve the recovery of sodium ions, phosphate ions and sulfate ions, and for lithium ions, they directly form sludge or are dried together with other impurity elements, and lithium is not effectively utilized.

[0004] Therefore, providing an environmentally friendly and economical method for efficiently recovering lithium from the synthesized washing water of iron phosphate is of great significance for the recovery and utilization of lithium resources and improving the economic and environmental benefits of treating the synthesized washing water of iron phosphate. Summary of the Invention

[0005] (1) Technical Problems to be Solved

[0006] In view of the deficiencies of the prior art, the present invention provides a method for recovering lithium from the washing water of synthesized iron phosphate by fully recovering all components of lithium iron phosphate black powder. Add the synthesized washing water to a sodium-containing alkaline substance, and sequentially carry out static settlement and separation to obtain a impurity-removing liquid and impurity-removing slag; carry out membrane concentration on the impurity-removing liquid to obtain concentrated water and fresh water; sequentially carry out extraction, carbon dioxide back-extraction for lithium precipitation, filtration, etc. on the concentrated water to obtain an extraction raffinate, lithium salt and lithium precipitation liquid. Return the lithium precipitation liquid to the previous extraction process, adjust the pH of the extraction raffinate and then evaporate and crystallize to obtain sodium sulfate crystals. Among them, the full-component recovery of lithium iron phosphate first extracts iron phosphate, which has stronger economic anti-risk ability, and the extraction method for recovering lithium from the washing water has a higher recovery rate and purer lithium salt. The fresh water obtained by membrane concentration in the present invention meets the pure water reuse standard and can be reused in the production of iron phosphate.

[0007] (2) Technical Solutions

[0008] To achieve the above object, the present invention is realized through the following technical solutions: A method for recovering lithium from the washing water in the synthesis of lithium iron phosphate by recycling all components of lithium iron phosphate black powder, specifically including the following steps:

[0009] S1. Add lithium iron phosphate black powder, sulfuric acid, and water into a reaction kettle. After a period of time, perform pressure filtration to separate the solid and liquid, obtaining a carbon residue and a mixed solution of lithium, iron, and phosphorus;

[0010] S2. Utilize the carbon residue for the resource utilization of graphite. Purify the mixed solution of lithium, iron, and phosphorus. Adjust the pH and the iron-to-phosphorus ratio to the standard with sodium carbonate and phosphoric acid. The mixed solution of lithium, iron, phosphorus, and sodium undergoes a synthesis reaction to synthesize amorphous iron phosphate;

[0011] S3. Perform pressure filtration to separate the solid and liquid. Remove impurities from the synthesis filtrate and send it to the lithium extraction process. The filter cake is slurried and washed and then sent to the iron phosphate process to obtain the washed synthesis washing water;

[0012] S4. Add a sodium-containing alkaline substance to the iron phosphate synthesis washing water, and sequentially perform static settling and separation to obtain an impurity-removing liquid and an impurity-removing residue;

[0013] S5. Perform membrane concentration on the impurity-removing liquid to obtain concentrated water and fresh water;

[0014] S6. Sequentially perform extraction, carbon dioxide back-extraction for lithium precipitation, and filtration on the concentrated water to obtain an extract residue, a lithium salt, and a lithium precipitation liquid;

[0015] S7. Return the lithium precipitation liquid to the previous extraction process. Adjust the pH of the extract residue and then perform evaporation crystallization to obtain sodium sulfate crystals.

[0016] Preferably, in step S3, the iron phosphate synthesis washing water is the washing water generated during the preparation of iron phosphate by the sodium method, and the iron phosphate synthesis washing water contains phosphate ions, sulfate ions, sodium ions, hydrogen ions, iron ions, and lithium ions.

[0017] Preferably, the concentration of sodium ions is 1.2 - 2.5 g / l, and the concentration of lithium ions is 500 - 800 mg / l.

[0018] Preferably, in step S3, the pH value of the washing water before adjustment is 2 - 3, and the pH after adjustment is 5 - 10.

[0019] Preferably, in step S4, the static settling time is 1 - 4 h, and the content of Fe in the impurity-removing liquid ≤ 5 ppm.

[0020] Preferably, in step S5, the pressure of membrane concentration is 3 - 12 MPa, and the pore diameter of the membrane < 1.5 nm.

[0021] Preferably, in step S5, the TDS of the concentrated water is 80 - 120 g / L, the lithium ion concentration is 2 - 3 g / l, the TDS of the fresh water < 10 mg / L, and the resistivity of the fresh water < 10 μS / cm.

[0022] Preferably, the extraction temperature in step S6 is 5-45°C, and the concentration of Li in the raffinate + is less than 20 ppm.

[0023] Preferably, in step S6, carbon dioxide is introduced for back-extraction, the back-extraction temperature is 5-45°C, and the time is 0.5-1 h.

[0024] Preferably, in the evaporation crystallization in step S7, the evaporator compresses the low-grade secondary steam through a mechanical steam compressor, raises the temperature and returns it as the evaporation heat source. The evaporation temperature is 90-95°C, the sodium salt is sodium sulfate, and the pH value of the sodium salt is 7-8.

[0025] (III) Beneficial effects

[0026] The present invention provides a method for recovering lithium from the synthesis phosphoric acid iron washing water by recycling all components of lithium iron phosphate black powder. Compared with the prior art, it has the following beneficial effects: the method for recovering lithium from the synthesis phosphoric acid iron washing water by recycling all components of lithium iron phosphate black powder adds a sodium-containing alkaline substance to the synthesis washing water, and sequentially performs static settling and separation to obtain an impurity-removing liquid and an impurity-removing residue; the impurity-removing liquid is subjected to membrane concentration to obtain concentrated water and fresh water; the concentrated water is sequentially subjected to extraction, carbon dioxide back-extraction for lithium precipitation, filtration, etc. to obtain a raffinate, a lithium salt and a lithium precipitation liquid. The lithium precipitation liquid is returned to the previous extraction process, and the raffinate is adjusted in pH and then subjected to evaporation crystallization to obtain sodium sulfate crystals. Among them, the economic anti-risk ability of recovering phosphoric acid iron first from all components of lithium iron phosphate is stronger, the lithium recovery rate by using extraction to recover lithium in the washing water is higher, and the lithium salt is purer. The fresh water obtained by membrane concentration in the present invention meets the pure water reuse standard and can be reused in the production of phosphoric acid iron. Brief description of the drawings

[0027] Figure 1 is the process flow chart of the present invention;

[0028] Figure 2 is the main process flow chart of impurity removal and concentration of lithium-containing washing water of phosphoric acid iron of the present invention. Detailed description of the invention

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] Please refer to Figure 1-2 , the embodiments of the present invention provide a technical solution: a method for recovering lithium from the synthesis phosphoric acid iron washing water by recycling all components of lithium iron phosphate black powder, which specifically includes the following steps:

[0031] Step 1: The black powder of lithium iron phosphate is leached with acid to remove impurities and synthesize amorphous iron phosphate. The amorphous iron phosphate is filtered and washed with water. An alkali solution containing sodium is added to the washing water for synthesizing iron phosphate, and then static settling and separation are carried out in sequence to obtain an impurity removal solution and impurity removal slag.

[0032] Step 2: The impurity removal solution is subjected to membrane concentration to obtain concentrated water and fresh water.

[0033] Step 3: The concentrated water is subjected to extraction, carbon dioxide back-extraction for lithium precipitation, filtration, etc. in sequence to obtain an extract raffinate, lithium salt and lithium precipitation solution.

[0034] Step 4: The lithium precipitation solution is returned to the previous extraction process, and the pH of the extract raffinate is adjusted and then evaporated and crystallized to obtain sodium sulfate crystals.

[0035] In the present invention, in Step 1, the washing water for synthesizing iron phosphate is the washing water generated during the recovery of black powder of lithium iron phosphate by the sodium method to prepare iron phosphate. The washing water for synthesizing iron phosphate contains sodium ions, phosphate ions, sulfate ions, hydrogen ions, iron ions and lithium ions.

[0036] In the present invention, in Step 1, the main components and leaching rate of the black powder of lithium iron phosphate are as Figure 2 shown, and the chemical index components are not limited to this table.

[0037] The components of the black powder are as shown in the following table:

[0038]

[0039] The leaching rate is as shown in the following table:

[0040] Component Li Fe P Cu AI C Ni Co Mn Ti F Others Total Acid leaching rate (%) 99％ 99％ 99％ 100％ 100％ 0 100％ 100％ 100％ 100％ / /

[0041] In the present invention, in Step 1, the main chemical reactions of the acid leaching of the black powder of lithium iron phosphate are as follows:

[0042] 2LiFePO 4 +3H 2 SO 4 =Li 2 SO 4 +2FeSO 4 +2H 3 PO 4 (1)

[0043] In the above reaction, the temperature is normal temperature, the solid-liquid ratio is 1:3 - 5, the pH after the reaction is 1.0 - 1.5, the acid washing and water washing liquid of the carbon slag is recycled to the acid leaching process, and the mass fraction of concentrated sulfuric acid is 98%.

[0044] In the present invention, in Step 1, the main components of the purified liquid after the acid leaching and impurity removal of the black powder of lithium iron phosphate are as shown in the following table:

[0045] Composition of the acid leaching impurity removal and purification liquid: (Fe / P = 0.97 - 1.03, PH = 2 - 2.5)

[0046] Component Li Fe P Al Cu Na F Density Total g / L 7.0 50.0 26.9 0.05 0 10.0 - 1.20

[0047] In the present invention, in step 1, the main chemical reactions for synthesizing iron phosphate from the acid leaching impurity removal and purification liquid are as follows:

[0048] 2FeSO 4 +H 2 O 2 +2H 3 PO 4 +2Na 2 CO 3 =2FePO 4 +2Na 2 SO 4 +2CO 2 +4H 2 O (2)

[0049] In the reaction, the concentration of hydrogen peroxide is 27.5%, the concentration of refined phosphoric acid is 85%, the sodium carbonate solution is 200 g / l, the solid content of the synthesized amorphous iron phosphate is 9% - 13%, the reaction PH = 1.5 - 2.2, the reaction time is 0.5 - 1 h, and the reaction temperature is 60 - 80 °C; the solid-liquid ratio of the washing water of the amorphous iron phosphate is 1:7 - 15, and the countercurrent washing mode is adopted.

[0050] In the present invention, in step 1, the main components of the washing water for synthesizing iron phosphate are shown in the following table:

[0051]

[0052]

[0053] In the present invention, in step 1, the pH value of the washing water for synthesizing iron phosphate is adjusted to 5 - 10.

[0054] In the present invention, in step 1, the standing time is 1 - 4 h, and the content of Fe in the impurity removal liquid ≤ 5 ppm.

[0055] In the present invention, in step 2, the pressure of the membrane concentration is 3 - 12 MPa, and the pore diameter of the membrane < 1.5 nm.

[0056] In the present invention, in step 2, the TDS of the concentrated water is 80 - 120 g / L, the lithium ion concentration is 2 - 3 g / l, the TDS of the fresh water < 10 mg / L, and the resistivity of the fresh water < 10 μS / cm. The main components of the concentrated water are shown in the following table:

[0057] Composition of the RO membrane concentrate of the washing water for phosphorus-iron synthesis

[0058] Water volume TDS Li Na <![CDATA[K + > <![CDATA[Ca 2+ > <![CDATA[Mg 2+ > <![CDATA[Al 3+ > Fe <![CDATA[m 3 / d]]> g / L mg / L mg / L mg / l mg / l mg / l mg / l mg / l Approximately 18.1 Approximately 84.40 2950.4 19042.2 25.6 131.1 6.6 6.6 20.0 <![CDATA[Ni 2+ > <![CDATA[Co 2+ > <![CDATA[Mn 2+ > <![CDATA[SO4 2- > <![CDATA[PO 4 3- > <![CDATA[Cl - > <![CDATA[F - > <![CDATA[NH 3- N]]> COD mg / l mg / l mg / l mg / l mg / l mg / l mg / l mg / l mg / l 295.0 131.1 229.5 61352.8 66.0 32.8 32.8 32.8 528.3

[0059] In the present invention, in step 1 and step 2, the main process flow chart of removing impurities and concentrating the lithium iron phosphate washing water is as follows: Figure 2 shown.

[0060] In the present invention, in step 3, the extraction temperature is 5 to 45°C, and the Li + The concentration is less than 20ppm, the introduction of carbon dioxide into the stripping system adopts a step-by-step introduction form, the pressure of the stripping system is 1-4MPa to introduce carbon dioxide, the stripping temperature is 5-45°C; the time is 0.5-1h, the volume ratio of the organic phase to the stripping agent is (50-10):1, the volume ratio of carbon dioxide to water in the stripping agent is (50-5):1, the extraction organic phase uses kerosene as a solvent, and contains any one or more of organic phosphorus oxide compounds, thiophenes, ketone compounds, alcohol compounds, pyrazoles, phosphates, quaternary ammonium salts, and amine compounds.

[0061] In the present invention, in step 3, the stripping solution is mainly a lithium bicarbonate solution, and the lithium bicarbonate solution is heated to 95° C. for pyrolysis for 2 hours, and after the heating is completed, it is filtered, washed, and dried to obtain battery-grade lithium carbonate;

[0062] In the present invention, in step 4, the lithium precipitation solution and the lithium carbonate washing water are returned to the front-end extraction process, and the raffinate is evaporated and crystallized after adjusting the pH value to obtain sodium sulfate crystals; the evaporator compresses the low-grade secondary steam through a mechanical steam compressor, and returns it as an evaporation heat source after increasing the temperature, the evaporation temperature is 90-95 degrees, and the pH value of the sodium salt is 7-8.

[0063] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0064] Example 1

[0065] Add 50t of concentrated sulfuric acid and 300t of carbon slag washing water (acid-containing) to 100t of lithium iron phosphate black powder. After 4 hours, separate the solid and liquid. First add medicaments to the filtrate, and then add sodium carbonate solution (17%) to adjust the pH value to 2.2 - 3 for impurity removal. Then add 85% refined phosphoric acid to adjust the iron-phosphorus ratio and the pH value to 2 - 2.5 to synthesize amorphous iron phosphate. After stirring and washing and sleeve washing, 6130t of synthetic washing water with a pH value of 2 - 3 is obtained; add 17wt% sodium carbonate solution to the synthetic washing water to adjust the pH value of the iron phosphate synthetic washing water to 6. After standing for 1h, pump the supernatant into a precision filter and an ultrafiltration device for filtration. Pump the lower turbid liquid into a plate and frame filter press for filtration and then pump it into a precision filter and an ultrafiltration device for further filtration. The obtained filtrate is an impurity removal liquid with a Fe content ≤ 5ppm; pass the impurity removal liquid through membrane concentration (working pressure is 10MPa, the pore size of the membrane is 1nm, and the type of the membrane is RO membrane) to obtain concentrated water with a TDS of 110g / L and fresh water with a TDS of 8mg / L and a resistivity of 8μS / cm; pump the concentrated water with a lithium content of 2 - 3g / l into an extraction device. The extraction temperature is 35°C, and the concentration of Li + in the raffinate is 11ppm. The temperature of carbon dioxide back-extraction is 25°C; the time is 30 minutes. Release the pressure of the liquid discharged from the back-extractor to obtain three phases of blank organic phase, concentrated lithium bicarbonate solution and CO 2 gas. The blank organic phase is returned to the extraction process, and the CO 2 gas is returned to the supercritical back-extraction process for recycling. After the concentrated lithium bicarbonate solution removes the entrained organic matter by oil removal, it is filtered and dried to obtain high-purity lithium carbonate, and battery-grade lithium carbonate is prepared; in the MVR evaporator of the raffinate, evaporate and concentrate and crystallize at 95°C to obtain sodium sulfate.

[0066] The comprehensive recovery rate of lithium in this example is 92%.

[0067] Example 2

[0068] Add 50t of concentrated sulfuric acid and 300t of carbon slag washing water (containing acid) to 100t of lithium iron phosphate black powder. After 4 hours, perform solid-liquid separation. First, add medicaments to the filtrate, and then add sodium carbonate solution (20%) to adjust the pH value to 2.2 - 3 for impurity removal. Then, add 85% refined phosphoric acid to adjust the iron-phosphorus ratio and the pH value to 2 - 2.5 to synthesize amorphous iron phosphate. After stirring and washing and sleeve washing, 6130t of synthetic washing water with a pH value of 2 - 3 is obtained; add 20wt% of sodium carbonate solution to the synthetic washing water to adjust the pH value of the iron phosphate synthetic washing water to 7. After standing for 2h, pump the upper clear liquid into a precision filter and an ultrafiltration device for filtration, and pump the lower turbid liquid into a plate and frame filter press for filtration and then into a precision filter and an ultrafiltration device for further filtration. The obtained filtrate is an impurity removal liquid with the Fe content ≤ 5ppm; pass the impurity removal liquid through membrane concentration (the working pressure is 12MPa, the pore size of the membrane is 1nm, and the type of the membrane is RO membrane) to obtain concentrated water with a TDS of 120g / L and fresh water with a TDS of 9mg / L and a resistivity of 8μS / cm; pump the concentrated water with a lithium content of 2 - 3g / l into an extraction device. The extraction temperature is 35°C, and the concentration of Li + in the raffinate is 11ppm. The temperature of carbon dioxide back-extraction is 25°C; the time is 30 minutes. Release the pressure of the liquid discharged from the back-extractor to obtain three phases of blank organic phase, concentrated lithium bicarbonate solution, and CO 2 gas. The blank organic phase is returned to the extraction process, and the CO 2 gas is returned to the supercritical back-extraction process for recycling. After the concentrated lithium bicarbonate solution is degreased to remove entrained organic matter, it is filtered and dried to obtain high-purity lithium carbonate, and battery-grade lithium carbonate is prepared; in the raffinate MVR evaporator, evaporate and concentrate and crystallize at 95°C to obtain sodium sulfate.

[0069] The comprehensive recovery rate of lithium in this example is 93%.

[0070] Comparative Example 1

[0071] Add 50t of concentrated sulfuric acid and 300t of carbon slag washing water (acid-containing) to 100t of lithium iron phosphate black powder. After 4 hours, perform solid-liquid separation. First add reagents to the filtrate, and then add sodium carbonate solution (17%) to adjust the pH value to 2.2 - 3 for impurity removal. Then add 85% refined phosphoric acid to adjust the iron-phosphorus ratio and the pH value to 2 - 2.5 to synthesize amorphous iron phosphate. After stirring and washing, 6130t of synthetic washing water with a pH value of 2 - 3 is obtained; add 17wt% sodium carbonate solution to the synthetic washing water to adjust the pH value of the iron phosphate synthetic washing water to 6. After standing for 1h, pump the supernatant into a precision filter and an ultrafiltration device for filtration. Pump the lower-layer turbid liquid into a plate-and-frame filter press for filtration and then pump it into a precision filter and an ultrafiltration device for further filtration. The obtained filtrate is an impurity-removing liquid with the Fe content ≤ 5ppm; subject the impurity-removing liquid to membrane concentration (working pressure is 10MPa, the pore size of the membrane is 5nm, and the type of the membrane is RO membrane) to obtain concentrated water with TDS of 110g / L and fresh water with TDS of 8mg / L and resistivity of 8μS / cm; pump the concentrated water with a lithium content of 2 - 3g / l into an extraction device. The extraction temperature is 35°C, and the concentration of Li + in the raffinate is 11ppm. The temperature of carbon dioxide back-extraction is 25°C; the time is 30 minutes. Release the pressure of the liquid discharged from the back-extractor to obtain three phases of blank organic phase, concentrated lithium bicarbonate solution, and CO 2 gas. The blank organic phase is returned to the extraction process, and the CO 2 gas is returned to the supercritical back-extraction process for recycling. After the concentrated lithium bicarbonate solution is defatted to remove entrained organic matter, it is filtered and dried to obtain high-purity lithium carbonate, and battery-grade lithium carbonate is prepared; in the MVR evaporator of the raffinate, evaporate and concentrate for crystallization at 95°C to obtain sodium sulfate.

[0072] The comprehensive recovery rate of lithium in this example is 62%.

[0073] Comparative Example 2

[0074] Add 50 t of concentrated sulfuric acid and 300 t of carbon slag washing water (acid-containing) to 100 t of lithium iron phosphate black powder. After 4 hours, perform solid-liquid separation. First, add reagents to the filtrate, and then add sodium carbonate solution (17%) to adjust the pH value to 2.2 - 3 for impurity removal. Then, add 85% refined phosphoric acid to adjust the iron-phosphorus ratio and the pH value to 2 - 2.5 to synthesize amorphous iron phosphate. After stirring and washing and sleeve washing, 6130 t of synthetic washing water with a pH value of 2 - 3 is obtained; add 17 wt% sodium carbonate solution to the synthetic washing water to adjust the pH value of the iron phosphate synthetic washing water to 6. After standing for 1 h, pump the supernatant into a precision filter and an ultrafiltration device for filtration. Pump the lower-layer turbid liquid into a plate and frame filter press for filtration and then pump it into a precision filter and an ultrafiltration device for further filtration. The obtained filtrate is an impurity-removing liquid with the Fe content ≤ 5 ppm; subject the impurity-removing liquid to membrane concentration (working pressure is 10 MPa, the pore size of the membrane is 2.5 nm, and the type of the membrane is RO membrane) to obtain concentrated water with a TDS of 110 g / L and fresh water with a TDS of 8 mg / L and a resistivity of 8 μS / cm; pump the concentrated water with a lithium content of 2 - 3 g / l into an extraction device. The extraction temperature is 35 °C, and the concentration of Li + in the raffinate is 11 ppm. The temperature of carbon dioxide back-extraction is 25 °C; the time is 30 minutes. Release the pressure of the liquid discharged from the back-extractor to obtain three phases of blank organic phase, concentrated lithium bicarbonate solution, and CO 2 gas. Return the blank organic phase to the extraction process, and return the CO 2 gas to the supercritical back-extraction process for recycling. After removing the entrained organic matter from the concentrated lithium bicarbonate solution by degreasing, filter and dry it to obtain high-purity lithium carbonate, and prepare battery-grade lithium carbonate; in the MVR evaporator of the raffinate, perform evaporation concentration and crystallization at 95 °C to obtain sodium sulfate.

[0075] The comprehensive recovery rate of lithium in this example is 70%.

[0076] From the above Examples 1 - 2 and Comparative Examples 1 - 2, it can be seen that the present invention provides a method for recovering lithium from the synthetic iron phosphate washing water of the whole components of lithium iron phosphate black powder. The recovery rate of lithium in the synthetic iron phosphate washing water exceeds 92%, achieving the effects of resource recovery and zero wastewater discharge. The economic value of the produced lithium carbonate and sodium sulfate products can make up for the cost of wastewater treatment. The present invention has good environmental and economic benefits.

[0077] Meanwhile, the content not described in detail in this specification belongs to the prior art well-known to those skilled in the art.

[0078] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device.

[0079] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for recovering lithium from all components of lithium iron phosphate black powder in synthetic iron phosphate washing water, characterized in that: The specific steps include: S1, adding lithium iron phosphate black powder, sulfuric acid and water into a reactor, and filtering the solid and liquid for a period of time to obtain a mixed solution of carbon slag and lithium iron phosphate; S2, carbon slag graphite resource utilization, lithium iron phosphorus mixed solution impurities removal, sodium carbonate and phosphoric acid to adjust the pH and iron phosphorus ratio to meet the standard, lithium iron phosphorus sodium mixed solution through synthesis reaction, synthesizing amorphous iron phosphate; S3, solid-liquid separation by filtration pressing, impurity removal and lithium extraction process of the synthetic filtrate, and iron phosphate removal process after filter cake slurry washing to obtain synthetic washing water; S4. Add sodium-containing alkaline substances to the iron phosphate synthesis washing water, and sequentially let it stand and separate to obtain impurity removal liquid and impurity removal residue; S5, concentrating the impurity removal liquid through a membrane to obtain concentrated water and fresh water; S6, extracting the concentrated water, stripping the lithium with carbon dioxide and filtering in sequence to obtain a raffinate, a lithium salt and a lithium precipitation solution; S7, returning the lithium precipitation solution to the previous extraction process, adjusting the pH of the raffinate solution and evaporating and crystallizing it to obtain sodium sulfate crystals.

2. The method for recovering lithium from the synthetic iron phosphate washing water from all components of lithium iron phosphate black powder according to claim 1, characterized in that: The iron phosphate synthesis washing water in step S3 is the washing water produced in the process of preparing iron phosphate by the sodium method, and the iron phosphate synthesis washing water contains phosphate ions, sulfate ions, sodium ions, hydrogen ions, iron ions and lithium ions.

3. The method for recovering lithium from synthetic iron phosphate washing water from all components of lithium iron phosphate black powder according to claim 2, characterized in that: The concentration of sodium ions is 1.2-2.5 g / l, and the concentration of lithium ions is 500-800 mg / l.

4. The method for recovering lithium from synthetic iron phosphate washing water from all components of lithium iron phosphate black powder according to claim 1, characterized in that: In step S3, the pH value of the washing water before adjustment is 2-3, and the pH value after adjustment is 5-10.

5. The method for recovering lithium from synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder according to claim 1, characterized in that: The standing time in step S4 is 1 to 4 hours, and the Fe content in the impurity removal liquid is ≤5ppm.

6. The method for recovering lithium from synthetic iron phosphate washing water from all components of lithium iron phosphate black powder according to claim 1, characterized in that: The pressure of membrane concentration in step S5 is 3-12 MPa, and the pore size of the membrane is less than 1.5 nm.

7. The method for recovering lithium from synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder according to claim 1, characterized in that: In step S5, the TDS of concentrated water is 80-120 g / L, the lithium ion concentration is 2-3 g / L, the TDS of fresh water is less than 10 mg / L, and the resistivity of fresh water is less than 10 μS / cm.

8. The method for recovering lithium from synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder according to claim 1, characterized in that: The extraction temperature in step S6 is 5 to 45°C, and the Li + The concentration is less than 20ppm.

9. The method for recovering lithium from synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder according to claim 1, characterized in that: In step S6, the stripping is performed by introducing carbon dioxide, and the stripping temperature is 5 to 45° C. and the stripping time is 0.5 to 1 h.

10. The method for recovering lithium from synthetic iron phosphate washing water by recovering all components of lithium iron phosphate black powder according to claim 1, characterized in that: In the evaporation crystallization in step S7, the evaporator compresses the low-grade secondary steam through a mechanical steam compressor, and returns it as an evaporation heat source after increasing the temperature. The evaporation temperature is 90-95 degrees, the sodium salt is sodium sulfate, and the pH value of the sodium salt is 7-8.

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