Method for preparing lithium iron phosphate at low cost through extraction-coprecipitation of lithium-containing brine

Abstract

The invention discloses a low-cost method for preparing lithium iron phosphate from lithium-containing brine through extraction-coprecipitation, and belongs to the technical field of lithium iron phosphate preparation. The problem that an organic solvent is easy to degrade in a high-concentration hydrochloric acid reverse extraction environment is solved. According to the method, lithium-containing brine, an iron source and an organic solvent are mixed, extracted and subjected to phase splitting, so that lithium and iron elements in the brine are co-extracted and enter an organic phase, after the organic phase is washed, a neutral solution or a dilute acid solution is adopted for carrying out reverse extraction operation on the loaded organic solvent, and lithium and iron in the organic solvent are co-extracted and enter a water phase; and the iron and lithium coexisting strip liquor is formed. The preparation method comprises the following steps: reducing iron ions into ferrous ions, adding a phosphorus source and a pH regulator to co-precipitate and separate out lithium iron phosphate, carrying out solid-liquid separation, washing and drying on a co-precipitated solid, adding a carbon source, and carrying out high-temperature calcination to obtain a lithium iron phosphate precursor; according to the invention, the problem of degradation of the organic solvent is effectively avoided, the supplement amount of the organic solvent is reduced, and low-cost and short-process preparation of lithium iron phosphate is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium iron phosphate preparation, and specifically relates to a method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine. Background Art

[0002] The efficient development and utilization of lithium resources is of great significance to the strategic development of the country's new energy emerging industries. Lithium iron phosphate is one of the key cathode materials for new energy lithium batteries, accounting for more than 70% of the lithium battery cathode material market. At present, the industrial production of lithium iron phosphate mainly adopts the solid phase method, which uses lithium carbonate and iron phosphate as the main raw materials and synthesizes lithium iron phosphate precursors through high-temperature calcination.

[0003] As a basic lithium salt, the industrial production of lithium carbonate mainly uses lithium-containing ores or salt lake brine as raw materials. After a series of separation and purification processes, lithium carbonate products are obtained through the precipitation reaction of sodium carbonate and lithium ions. The extraction method is one of the important methods for extracting lithium from salt lake brine. This method usually uses tributyl phosphate (TBP) as an extractant and ferric chloride as a co-extractant. It can effectively achieve a highly selective separation of lithium and impurity elements in brine, and is particularly suitable for brine systems with a high magnesium-lithium ratio. During the extraction process of lithium, lithium is mainly present in the organic phase in the form of TBP·Li[FeCl4]. The high chlorine environment can promote the conversion of ferric chloride into a stable [FeCl4]- complex anion. Therefore, the chloride ion concentration of the brine in the extraction system is usually required to reach about 6 mol / L. Since TBP·H[FeCl4] is more stable than TBP·Li[FeCl4], the lithium-loaded organic solvent is usually stripped with hydrochloric acid, and the hydrochloric acid concentration is usually required to reach about 6 mol / L, otherwise the iron ions will be stripped into the stripping solution together, affecting the purity of lithium carbonate in the subsequent lithium precipitation process. However, high concentration of hydrochloric acid can easily cause degradation of organic solvents, which has a serious impact on the stability and economy of the extraction process.

[0004] In general, the current lithium iron phosphate industry chain uses lithium carbonate as an intermediate material. The lithium precipitation process consumes a large amount of sodium carbonate, and a large amount of carbon dioxide emissions are generated during the solid-phase synthesis of lithium iron phosphate. At the same time, in order to avoid the impact of iron on lithium carbonate products, the extraction method of lithium extraction requires high-concentration hydrochloric acid for stripping, which has a serious impact on the stability of the process. In addition, high-concentration hydrochloric acid can easily cause organic solvent degradation, resulting in a large amount of organic solvent addition and high cost. In addition, the current single-pass lithium yield of the industrial lithium precipitation process is only about 80%, and it is necessary to further recover the remaining 20% ​​of lithium resources in the lithium precipitation mother liquor through evaporation crystallization, solvent extraction and other means, resulting in low overall operating efficiency of industrial production. Summary of the invention

[0005] In view of the problems existing in the preparation process of lithium iron phosphate in the prior art, the present invention provides a method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine, which not only solves the problem of organic solvent stability in the high-concentration hydrochloric acid stripping environment of the traditional extraction and lithium extraction process, reduces the solvent regeneration section in the traditional extraction and lithium extraction process, but also reduces the material consumption and energy consumption in the preparation process of lithium iron phosphate positive electrode materials, reduces carbon dioxide emissions, and promotes the green and low-carbon upgrading and development of the lithium iron phosphate positive electrode materials industry.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine, comprising the following steps:

[0008] S1: adding iron salt to lithium-containing brine to obtain a mixture, subjecting the mixture to multi-stage countercurrent extraction with an organic solvent to obtain an organic solvent loaded with both lithium and iron;

[0009] S2: removing impurities from the organic solvent obtained in S1;

[0010] S3: stripping the organic solvent obtained in S2 to obtain a stripping solution containing both iron and lithium;

[0011] S4: adjusting the lithium-iron ratio of the stripping solution obtained in S3 to a set range;

[0012] S5: reducing the iron ions in the stripping solution obtained in S4 to ferrous ions;

[0013] S6: adding a phosphorus source to the stripping solution obtained in S5, and co-precipitating lithium iron phosphate by adjusting the pH;

[0014] S7: performing solid-liquid separation on the solid-liquid mixture obtained in S6, washing and drying the obtained solid precipitate, adding a carbon source and then calcining at high temperature to obtain a lithium iron phosphate precursor;

[0015] S8: Obtain finished lithium iron phosphate based on the lithium iron phosphate precursor obtained in S7.

[0016] Preferably, in S6, the pH is adjusted to alkaline so that phosphorus, iron and lithium in the stripping solution are co-precipitated.

[0017] After adopting this technical solution, the pH is adjusted to alkaline for two purposes. One is to allow lithium iron phosphate to precipitate more completely. Although the solubility of lithium iron phosphate in dilute acid is very low, alkalinity is more conducive to its precipitation. Another reason is that if the acidic solution is dehydrated and heated, the corrosion of the equipment will be very serious, and expensive acid-resistant materials need to be selected. If it is alkaline, the requirements for materials will be relatively low, and the corrosion will be much less, which can effectively increase the service life of the equipment and reduce production costs.

[0018] Preferably, the pH is adjusted to 8-10 in S6.

[0019] Preferably, in S2, the organic solvent is washed with a washing liquid to remove impurities in the organic solvent.

[0020] Preferably, in S2, the organic solvent is washed with a washing liquid to remove impurities in the organic solvent, and the washing liquid is one or more of an acid solution, an aqueous solution, an aqueous solution containing lithium, an aqueous solution containing an inorganic iron salt, an aqueous solution containing both lithium and an inorganic iron salt, a hydrothermal synthesis filtrate, and a mother liquor after the hydrothermal synthesis filtrate is concentrated and crystallized, and when the washing liquid is an acid solution, H + The concentration is less than 2.0 mol / L.

[0021] Preferably, the washing liquid after washing is returned to the lithium-containing brine in S1.

[0022] Preferably, in S4, the lithium-iron molar ratio is adjusted to be greater than 1.01 by adding an external lithium source, and the added lithium source is an inorganic lithium source, such as lithium chloride, lithium sulfate, lithium nitrate, etc.

[0023] Preferably, the phosphorus source added in S6 is one or more of an ammonium phosphate phosphorus source (such as ammonium phosphate, monoammonium hydrogen phosphate, diammonium phosphate, etc.), a sodium phosphate salt phosphorus source (such as sodium phosphate dodecahydrate, sodium dihydrogen phosphate, disodium hydrogen phosphate, etc.) and a potassium phosphate salt phosphorus source (such as potassium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, etc.), and the molar ratio of phosphate to ferrous ion after adding the phosphorus source is 0.5-1.0, and the pH can be adjusted by ammonia water, sodium hydroxide, potassium hydroxide, etc., and the pH value is greater than 7 after adjusting the pH.

[0024] Preferably, the reducing agent added in S4 is an organic reducing agent or an inorganic reducing agent such as sodium thiosulfate, an organic reducing agent such as ascorbic acid, citric acid, oxalic acid, etc., and the molar ratio of the reducing agent to the iron ion is greater than 1.01.

[0025] Preferably, the organic solvent in S1 includes an extractant and a diluent (which may be sulfonated kerosene or common low-viscosity diluents such as aromatic hydrocarbons, esters, ethers, etc.), and the extractant is an organic phosphorus solvent (such as tributyl phosphate, dibutyl phosphate, etc.) and / or a ketone solvent (such as methyl isobutyl ketone, diisobutyl ketone, etc.), the iron salt is an inorganic iron salt (such as ferric chloride, ferric sulfate, ferric nitrate, etc.), and the iron-to-lithium molar ratio in the lithium-containing brine is not less than 1 after the iron salt is added.

[0026] Preferably, the organic solvent obtained in S2 is stripped by stripping liquid in S3, wherein the stripping liquid is the filtrate of hydrothermal synthesis in S7, the mother liquor after the filtrate of thermal synthesis is concentrated and crystallized, the aqueous solution, H + One or more dilute acid solutions with a concentration of less than 2.0 mol / L.

[0027] Preferably, the carbon source added in S7 is one or more of carbon black, glucose, sucrose, starch, and citric acid. The high-temperature calcination synthesis temperature is 500-1000°C, and the calcination time is not less than 2 hours. After calcination, a lithium iron phosphate precursor can be obtained.

[0028] Preferably, in S8, the lithium iron phosphate precursor obtained in S7 can be further crushed and subjected to conventional treatment according to actual product performance requirements to obtain the lithium iron phosphate product. The inorganic salt resources in the filtrate after separation can be further recovered after concentration and crystallization. The mother liquor can be circulated to the washing section and the stripping section after adjusting the pH to neutral or weakly acidic and performing oxidation treatment.

[0029] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0030] The present invention uses lithium-containing brine to directly prepare lithium iron phosphate, which changes the original process of using 6 mol / L hydrochloric acid for lithium stripping in the original brine extraction and lithium extraction, and directly uses dilute acid or water or related salt solutions for iron and lithium co-extraction, and uses the stripping solution in which iron and lithium coexist to directly hydrothermally prepare lithium iron phosphate. The related technology not only solves the problem of organic solvent degradation caused by high-concentration hydrochloric acid during the operation of the original extraction and lithium extraction process, but also reduces the amount of organic solvent added in this process to about 5% compared to the annual organic solvent addition of more than 20% in the traditional solvent extraction and lithium extraction process, significantly improving the stability and economy of the brine lithium resource utilization process. At the same time, the existing overall process does not involve the synthesis of the basic lithium salt lithium carbonate, avoids the consumption of sodium carbonate and carbon dioxide emissions, and realizes the green, low-carbon and short-process preparation of lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a process flow chart of the present invention;

[0032] Figure 2 The XRD pattern of the lithium iron phosphate precursor prepared in Example 1;

[0033] Figure 3 This is a scanning electron microscope image of the lithium iron phosphate precursor prepared in Example 1, wherein Figure 3 A is a scanning electron microscope image at a magnification of 50,000 times. Figure 3 B is a scanning electron microscope image at a magnification of 100,000 times. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application.

[0035] It should be noted that the lithium-containing brine used in the following examples and comparative examples specifically includes salt lake brine, geothermal water, gas field produced water, and lithium-containing wastewater generated in industrial production processes, etc., and its common impurity elements mainly include sodium, potassium, magnesium, calcium and other elements, and anions mainly include chloride ions, sulfate ions, etc. If the chloride ion concentration in the original brine is insufficient, the chloride ion concentration can be increased to about 6 mol / L by concentration, addition of magnesium chloride, etc., and ensure that the brine is acidic or neutral to ensure the extraction rate of lithium.

[0036] Example 1

[0037] like Figure 1 As shown, a method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine comprises the following steps:

[0038] S1: Take 5L of lithium-containing brine, whose specific composition is 2.512g / L of lithium ions, 5.740g / L of sodium ions, 5.155g / L of potassium ions, 85.83g / L of magnesium ions, and 216.1g / L of chloride ions, and add ferric chloride to the lithium-containing brine at a lithium-iron molar ratio of 1.05:1, and then perform six-stage countercurrent extraction with an organic solvent (composed of TBP and sulfonated kerosene, with a TBP volume fraction of 80% and a sulfonated kerosene volume fraction of 20%) and the lithium-containing brine at an oil-water volume ratio of 1:1 to obtain an organic solvent loaded with lithium and iron at the same time, and the lithium extraction rate reaches 99.5%;

[0039] S2: Prepare 1 mol / L diluted hydrochloric acid washing liquid, and wash the organic solvent loaded with lithium and iron at an oil-water volume ratio of 40:1 to further reduce the entrainment of sodium, potassium and magnesium elements in the organic solvent, and the impurity ion washing removal rate is greater than 90%; the washing liquid after washing is returned to the lithium-containing brine in the S1 section;

[0040] S3: Prepare 0.1 mol / L hydrochloric acid stripping solution, and strip the washed loaded organic solvent at a volume ratio of 1:1 between oil and water to obtain a stripping solution containing both iron and lithium, with a lithium ion stripping rate of 99.0%;

[0041] S4: adjusting the lithium iron molar ratio by adding an inorganic lithium salt such as lithium chloride until the lithium iron molar ratio is 3;

[0042] S5: adding ascorbic acid at twice the molar equivalent of the iron ion reaction, and reducing the iron ions in the stripping solution obtained in S4 to ferrous ions for 30 minutes, with a reduction reaction conversion rate of 99.9%;

[0043] S6: adding ammonium dihydrogen phosphate solution in an equivalent molar amount of ferrous ions, adjusting the pH of the solution to 9 with aqueous ammonia, so that lithium iron phosphate in the stripping solution is co-precipitated to form a solid-liquid mixture, and performing solid-liquid separation;

[0044] S7: The obtained solid precipitate is washed three times with deionized water, and the washed solid precipitate is dried and dehydrated in a 60°C oven, 10% by mass of glucose is added, and after grinding and mixing, it is calcined in an inert atmosphere at 700°C in a tube furnace for 8 hours to obtain a lithium iron phosphate precursor product, whose XRD is as follows: Figure 2 As shown; after separation, the filtrate can be further recovered after concentration and crystallization, and the mother liquor can be circulated to S2 and S3 after adjusting the pH to neutral or weakly acidic;

[0045] S8: Based on the lithium iron phosphate precursor obtained in S7, the lithium iron phosphate precursor obtained in S7 can be further crushed and subjected to conventional treatment according to actual product performance requirements to obtain a lithium iron phosphate finished product.

[0046] The electron microscope scanning image of the lithium iron phosphate obtained in this embodiment is as follows Figure 3 As shown, from Figure 3 It can be seen that lithium iron phosphate products are mainly in layered and flaky form, and at high magnification, some rod-shaped particles can be seen attached to the surface.

[0047] Example 2

[0048] A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine, comprising the following steps:

[0049] S1: Take 5L of lithium-containing brine, whose specific composition is 2.512g / L of lithium ions, 5.740g / L of sodium ions, 5.155g / L of potassium ions, 85.83g / L of magnesium ions, and 216.1g / L of chloride ions, and add ferric chloride to the lithium-containing brine at a lithium-iron molar ratio of 1.1:1, and then perform six-stage countercurrent extraction with an organic solvent (composed of TBP and sulfonated kerosene, with a TBP volume fraction of 80% and a sulfonated kerosene volume fraction of 20%) and the lithium-containing brine according to an oil-water volume ratio of 1:1 to obtain an organic solvent loaded with lithium and iron at the same time, and the lithium extraction rate reaches 98.9%;

[0050] S2: At the beginning of operation, a 1 mol / L dilute hydrochloric acid washing liquid is prepared to wash the organic solvent loaded with lithium and iron at an oil-water volume ratio of 30:1 to further reduce the entrainment of sodium, potassium and magnesium elements in the organic solvent, and the impurity ion washing removal rate is greater than 90%; the washing liquid after washing is returned to the lithium-containing brine in the S1 section;

[0051] S3: In the initial state, 0.1 mol / L hydrochloric acid stripping solution is prepared. After a period of operation, the mother liquor obtained by evaporation and crystallization of the mother liquor of lithium iron phosphate hydrothermal synthesis at the end of the process is used, and the pH of the evaporated crystallization mother liquor is adjusted to 6, and ozone oxidation is introduced as the stripping solution, and the washed loaded organic solvent is stripped at a volume ratio of 1:1 between oil and water to obtain a stripping solution containing both iron and lithium, with a lithium ion stripping rate of 99.3%;

[0052] S4: Determine whether an additional lithium source is needed based on the lithium-iron ratio of the stripping solution. For example, excessive lithium ions have been added in the process startup stage and are stored in the evaporated crystallization mother liquor, and the crystallization mother liquor is all circulated as the stripping solution. At this time, there is sufficient lithium source in the stripping solution, so there is no need to add additional lithium source. However, if part of the crystallization mother liquor is used as the washing liquid in S2, and only another part of the crystallization mother liquor is used as the stripping solution, then additional lithium source needs to be added. When adding the lithium source, the lithium-iron molar ratio is adjusted by adding inorganic lithium salts such as lithium chloride until the lithium-iron molar ratio is not less than 1.01. In this embodiment, the lithium-iron molar ratio is adjusted to 3;

[0053] S5: adding ascorbic acid at 1.5 times the molar equivalent of the iron ion reaction, and reducing the iron ions in the stripping solution obtained in S4 to ferrous ions for 30 minutes, with a reduction reaction conversion rate of 99.9%;

[0054] S6: adding ammonium dihydrogen phosphate solution in an equivalent molar amount of ferrous ions, adjusting the pH of the solution to 9 with aqueous ammonia, so that lithium iron phosphate in the stripping solution is co-precipitated to form a solid-liquid mixture, and performing solid-liquid separation;

[0055] S7: Solid-liquid separation. The filtrate is evaporated and crystallized to obtain the by-product ammonium chloride product. The pH of the evaporation and crystallization mother liquor is adjusted to 6. After ozone oxidation treatment, it is further recycled to the stripping section. The obtained solid precipitate is washed three times with deionized water. After washing, the solid precipitate is dried and dehydrated in a 60°C oven, 10% by mass of glucose is added, and after grinding and mixing evenly, it is calcined in an inert atmosphere at a high temperature of 700°C in a tubular furnace for 8 hours to obtain a lithium iron phosphate precursor product. Its XRD is as follows Figure 2 As shown; after separation, the filtrate can be further recovered after concentration and crystallization, and the mother liquor can be circulated to S2 and S3 after adjusting the pH to neutral or weakly acidic;

[0056] S8: Based on the lithium iron phosphate precursor obtained in S7, the lithium iron phosphate precursor obtained in S7 can be further crushed and subjected to conventional treatment according to actual product performance requirements to obtain a lithium iron phosphate finished product.

[0057] The parameter changes of the other embodiments and comparative examples are shown in Table 1. It should be noted that the same points as those of other embodiments or comparative examples as those of Example 1 are not filled in:

[0058] Table 1

[0059]

[0060]

[0061]

[0062] The relevant result data of Examples 1-9 and Comparative Examples 1-4 are shown in Table 2:

[0063] Table 2

[0064]

[0065] It can be seen from Table 1 and Table 2 that compared with the prior art, the present invention not only realizes the short-process preparation of lithium iron phosphate, but also effectively reduces the degradation rate of organic solvents, reduces the amount of organic solvents to be added, and reduces production costs.

[0066] The above-mentioned embodiments only express the specific implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the protection scope of the present application. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the technical solution concept of the present application, and these all belong to the protection scope of the present application.

Claims

1. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine, characterized in that: The following steps are involved: S1: adding iron salt to lithium-containing brine to obtain a mixture, subjecting the mixture to multi-stage countercurrent extraction with an organic solvent to obtain an organic solvent loaded with both lithium and iron; S2: removing impurities from the organic solvent obtained in S1; S3: stripping the organic solvent obtained in S2 to obtain a stripping solution containing both iron and lithium; S4: adjusting the lithium-iron ratio of the stripping solution obtained in S3 to a set range; S5: reducing the iron ions in the stripping solution obtained in S4 to ferrous ions; S6: adding a phosphorus source to the stripping solution obtained in S5, and adjusting the pH to co-precipitate the phosphorus, iron and lithium elements in the stripping solution; S7: performing solid-liquid separation on the solid-liquid mixture obtained in S6, washing and drying the obtained solid precipitate, adding a carbon source and then calcining at high temperature to obtain a lithium iron phosphate precursor; S8: obtaining finished lithium iron phosphate based on the lithium iron phosphate precursor obtained in S7; The order between S4 and S5 can be swapped.

2. The method for preparing lithium iron phosphate by extraction-coprecipitation of lithium-containing brine at low cost according to claim 1, characterized in that: In S6, the pH is adjusted to alkaline so that phosphorus, iron and lithium in the stripping solution are co-precipitated.

3. The method for preparing lithium iron phosphate by extraction-coprecipitation of lithium-containing brine at low cost according to claim 1, characterized in that: In S2, the organic solvent is washed with a washing liquid to remove impurities in the organic solvent, wherein the washing liquid is one or more of an acid solution, an aqueous solution, an aqueous solution containing lithium, an aqueous solution containing an inorganic iron salt, an aqueous solution containing both lithium and an inorganic iron salt, a hydrothermal synthesis filtrate, and a mother liquor after the hydrothermal synthesis filtrate is concentrated and crystallized, and when the washing liquid is an acid solution, H + The concentration is less than 2.0 mol / L.

4. The method for preparing lithium iron phosphate by extraction-coprecipitation of lithium-containing brine at low cost according to claim 3, characterized in that: The washing liquid after washing is returned to the lithium-containing brine in S1.

5. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: In S4, the lithium-iron molar ratio is adjusted to be greater than 1.01 by adding an external lithium source, and the added lithium source is an inorganic lithium source.

6. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: The phosphorus source added in S6 is one or more of an ammonium phosphate phosphorus source, a sodium phosphate phosphorus source and a potassium phosphate phosphorus source, and after adding the phosphorus source, the molar ratio of phosphate to ferrous ion is 0.5-1.0, and the pH value is greater than 7 after adjusting the pH.

7. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: The reducing agent added in S4 is an organic reducing agent, and the molar ratio of the organic reducing agent to the iron ions is greater than 1.

01.

8. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: The organic solvent described in S1 includes an extractant and a diluent, and the extractant is an organic phosphorus solvent and / or a ketone solvent, the iron salt is an inorganic iron salt, and after the iron salt is added, the molar ratio of iron to lithium in the lithium-containing brine is not less than 1.

9. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: In S3, the organic solvent obtained in S2 is back-extracted using a back-extracting liquid, wherein the back-extracting liquid is the filtrate of the hydrothermal synthesis in S7, the mother liquor after the filtrate of the thermal synthesis is concentrated and crystallized, the aqueous solution, H + One or more dilute acid solutions with a concentration of less than 2.0 mol / L.

10. A method for preparing lithium iron phosphate at low cost by extraction-coprecipitation of lithium-containing brine according to any one of claims 1 to 4, characterized in that: The carbon source added in S7 is one or more of carbon black, glucose, sucrose, starch, and citric acid. The high-temperature calcination synthesis temperature is 500-1000°C, and the calcination time is not less than 2 hours. The calcined product can be crushed to obtain a lithium iron phosphate precursor.

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