Method for preparing battery-grade lithium dihydrogen phosphate by using waste lithium molecular sieve

Abstract

The invention belongs to the technical field of inorganic material preparation, and particularly relates to a method for preparing battery-grade lithium dihydrogen phosphate by using a waste lithium molecular sieve, which comprises the following steps: grinding the waste lithium molecular sieve, sieving with a 100-mesh sieve, mixing with a diammonium hydrogen phosphate solution, adding oxalic acid solid, adjusting the pH value of slurry to 5-6, stirring, and filtering to obtain filter residue and filtrate; performing the operations on the filter residue, filtering, and mixing with the first filtrate to obtain a solution 1; adding EDTA (Ethylene Diamine Tetraacetic Acid) into the solution 1, stirring, adding lithium hydroxide monohydrate, adjusting the pH value to 12.5-13, heating until boiling, and filtering while hot to obtain a lithium phosphate wet material; then adding pure water to obtain slurry, adding the slurry into phosphoric acid and hydrogen peroxide, and dissolving and clarifying to obtain a lithium dihydrogen phosphate clean solution; and then evaporating, stopping heating when the temperature of the solution reaches 125-130 DEG C, cooling, and then centrifuging and drying to obtain the battery-grade lithium dihydrogen phosphate. According to the invention, zero discharge of waste liquid is realized, and the lithium yield is high.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic material preparation, and in particular relates to a method for preparing battery-grade lithium dihydrogen phosphate by utilizing waste lithium molecular sieves. Background Art

[0002] Lithium molecular sieve is mainly used in pressure swing adsorption oxygen production equipment. After repeated use, the adsorption performance of lithium molecular sieve decreases. Considering economic benefits, it needs to be eliminated. In recent years, with the growth of demand for household and medical oxygen, a large number of lithium molecular sieves have been used to produce oxygen, which has led to a large amount of waste lithium molecular sieves produced every year. Lithium molecular sieve is a lithium X-type aluminosilicate crystal, in which the lithium content accounts for about 4%, which is much higher than the lithium content in spodumene and lepidolite, two lithium ores, and has a high recycling value. Since lithium molecular sieve is an aluminosilicate, this substance is easily destroyed by acid or alkali, resulting in the lithium, silicon, and aluminum in the solution in a free state. In the process of lithium carbonate precipitation, a large amount of silicon and aluminum impurities are wrapped in the product, resulting in low product purity.

[0003] For example, patent application CN116179857A discloses a method for extracting lithium from waste lithium molecular sieves, wherein the ground and sieved lithium molecular sieves are leached with a 20-35% sodium salt solution (filter residue is rinsed with pure water after filtration, and washing liquid is returned to the previous level to prepare sodium salt solution), the leached liquid is evaporated and concentrated after impurities are removed, sodium carbonate is added to precipitate lithium carbonate, and then washed and dried twice to obtain a lithium carbonate product. The washing waste liquid generated in this process is relatively large, and a small amount of lithium is contained in the waste liquid, resulting in lithium loss; in addition, the leached salt solution cannot be recycled (since sodium chloride is soluble, there is little sodium chloride in the filter residue, and after washing the filter residue, the sodium chloride in the washing liquid is even less, and each time a 20-35% sodium salt solution is prepared, it is basically necessary to add sodium chloride solid again, and a large amount of sodium chloride is contained in the filtrate. In order to obtain industrial lithium carbonate, this part of sodium chloride cannot be used, and it is also necessary to try to remove it), resulting in a waste of resources, and the process can only obtain industrial carbon, and other lithium salts with higher purity cannot be directly obtained.

[0004] Therefore, how to selectively extract lithium from waste lithium molecular sieves and obtain high-purity lithium salts has become a technical challenge. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing battery-grade lithium dihydrogen phosphate by utilizing waste lithium molecular sieves. By adopting secondary salt solution leaching, battery-grade lithium dihydrogen phosphate can be directly prepared from the lithium molecular sieve without passing through industrial-grade lithium carbonate (industrial carbon), thereby achieving zero discharge of waste liquid, high lithium yield, and the leaching salt solution can be recycled.

[0006] To achieve the above object, the present invention provides a method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves, comprising the following steps:

[0007] Step 1: Grind and sieve the waste lithium molecular sieve and mix it with diammonium phosphate solution, wherein the NH 4 + and Li + The molar ratio of is (3.5-4.5):1, and then immediately adding oxalic acid solid, adjusting the slurry pH to 5-6, stirring for a preset time, and filtering to obtain a filter residue and a filtrate;

[0008] Step 2: subjecting the filter residue to the operation of step 1 above, and mixing the filtrate obtained after filtration with the filtrate in step 1 to obtain solution 1;

[0009] Step 3: According to the Mg in solution 1 2+ concentration, adding EDTA to the solution 1, stirring for a preset time, adding lithium hydroxide monohydrate, adjusting the pH value to 12.5-13, and then heating to boiling, and filtering while hot after boiling for a preset time to obtain lithium phosphate wet material; wherein, the EDTA and Mg 2+ The molar ratio is (1-1.5):1;

[0010] Step 4: Detecting Mg in the lithium phosphate wet material 2+ and C 2 O 4 2- The impurity content and Li 3 PO 4 The content of lithium phosphate is then added into pure water to obtain a slurry, and the slurry is mixed with phosphoric acid and hydrogen peroxide, wherein Li 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of the lithium phosphate wet material to the pure water is 1: (2-2.1), and the mass ratio of the lithium phosphate wet material to the pure water is 1: (1.85-1.9), and the H 2 O 2 and C 2 O 4 2- The molar ratio is (2-5):1, and after the slurry is dissolved, a lithium dihydrogen phosphate clean solution is obtained;

[0011] Step 5: Heat the lithium dihydrogen phosphate clean solution to evaporate, stop heating when the solution temperature reaches 125-130° C., cool to a preset temperature, and then centrifuge and dry to obtain battery-grade lithium dihydrogen phosphate.

[0012] Furthermore, in step 1, the mass concentration of the diammonium hydrogen phosphate solution is 30-40%.

[0013] Furthermore, in step 1, the sieving is through a 100-mesh sieve; and the preset stirring time is 1-3 hours.

[0014] Furthermore, in step 3, the preset stirring time is 10-15 minutes.

[0015] Furthermore, in step 3, the preset boiling time is 5-10 minutes.

[0016] Furthermore, in step 3, the filtered filtrate and the ammonia volatilized during the heating evaporation process are introduced into phosphoric acid to form a diammonium hydrogen phosphate solution which is evenly divided and returned to step 1 and step 2 for repeated use.

[0017] Furthermore, in step 3, the mass concentration of the introduced phosphoric acid is 83-88%, preferably 85%.

[0018] Furthermore, in step 4, the mass fraction of the phosphoric acid is 83-88%, preferably 85%; the mass concentration of the hydrogen peroxide is 28-33%, preferably 30%.

[0019] Furthermore, in step 5, the preset temperature is 35-40°C.

[0020] Furthermore, in step 5, the mother liquor obtained by centrifugation enters step 4 to replace part of the pure water for reuse.

[0021] In some specific embodiments, a method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0022] Step 1: Take 200g of waste lithium molecular sieve, grind it through a 100-mesh sieve and mix it with 30-40wt% diammonium hydrogen phosphate solution, where n(NH 4 + ):n(Li + )=(3.5-4.5):1, immediately add oxalic acid solid, adjust the slurry pH to 5-6, stir for 1-3h, and filter to obtain a filter residue and a filtrate;

[0023] Step 2: The filter residue is filtered through the above steps and mixed with the filtrate in step 1 to obtain solution 1, and the concentrations of Ca, Mg, Si and Al impurities in solution 1 are detected; wherein Ca, Mg and Al mainly exist in the form of ions, and Si mainly exists in the form of SiO 2 The form of precipitation, only a very small amount of slightly soluble silicon-containing substances exists in the solution;

[0024] Step 3: Add EDTA to solution 1 to satisfy n(EDTA):n(Mg 2+)=(1-1.5):1, after stirring for 10-15min, add lithium hydroxide monohydrate, adjust the pH to 12.5-13 and heat to boiling, boil for 5-10min and filter while hot to obtain wet lithium phosphate, pass the filtered filtrate and the ammonia volatilized during the evaporation process into 85%wt phosphoric acid to obtain diammonium hydrogen phosphate solution, and divide the diammonium hydrogen phosphate solution equally for step 1 and step 2;

[0025] Step 4: Detection of Mg in lithium phosphate wet material 2+ , C 2 O 4 2- The impurity content is then added into pure water to obtain a slurry, and the slurry is added into 85wt% phosphoric acid and 30wt% hydrogen peroxide to meet the requirements of Li 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:(2-2.1); and the mass ratio of H to pure water is 1:(1.85-1.9), n(H 2 O 2 ):n(C 2 O 4 2- )=(2-5):1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0026] Step 5: Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 125-130°C, cool to 35-40°C, then centrifuge and dry to obtain battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0027] In general, the above technical solutions conceived by the present invention have the following technical advantages and beneficial effects compared with the prior art:

[0028] 1. The method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves provided by the present invention adopts diammonium hydrogen phosphate for leaching, then adds oxalic acid to remove most of the divalent and trivalent metal ions, and then adds EDTA to remove Mg in the solution. 2+ Then, the pH value was adjusted to 12.5-13 by LiOH to form Li 3 PO 4 Precipitation was achieved by adding H 3 PO 4 Make 3 PO 4 The precipitate was dissolved by H 2 O 2 Removal of excess C 2 O 4 2-, high-purity lithium dihydrogen phosphate is obtained. The present invention achieves zero discharge of waste liquid, is environmentally friendly, and has high lithium yield and purity.

[0029] 2. By using diammonium hydrogen phosphate for leaching, ammonia can be reused through heating recovery, and phosphate as a component of the product can also be fully utilized, saving resources.

[0030] 3. The present invention not only recycles waste lithium molecular sieves and turns waste into treasure, but also can directly produce battery-grade lithium dihydrogen phosphate from lithium molecular sieves without industrial carbon (lithium carbonate is a basic lithium salt, and other lithium salts can be produced from lithium carbonate. The present invention completely bypasses lithium carbonate. Generally, lithium salt recovery or lithium extraction from salt lakes and ores is to produce crude carbon or industrial carbon, while the present invention can directly produce lithium dihydrogen phosphate from recovery), with higher purity and greater benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of the method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves of the present invention;

[0032] Figure 2 This is the distribution diagram of the morphology of phosphorus-containing particles in phosphate solution at different pH at room temperature. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0034] The process principle of the method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves provided by the present invention is:

[0035] 1. Lithium ion molecular sieve (lithium ion molecular sieve is mainly composed of silicon oxygen tetrahedron and aluminum oxygen tetrahedron, and the aluminum ions are replaced by lithium ions through ion exchange to form lithium ion molecular sieve) after grinding and mixing with (NH 4 ) 2 HPO 4 The solution was mixed and oxalic acid H was immediately added 2 C 2 O 4 , adjust pH to 5-6, phosphate mainly in the form of H 2 PO 4 - Oxalate precipitates divalent metal ions and trivalent metal ions (a small amount of monovalent metal ions remaining in the solution, excess oxalate, H 2 PO 4- And a small amount of HPO 4 2- ).like Figure 2 The distribution of phosphorus-containing particles in phosphate solution at different pH values ​​at room temperature, where the pH values ​​corresponding to points a, b, and c are 2.12, 7.21, and 12.36, respectively. When the pH value is 0 to 2.12, phosphorus is mainly present in the form of H 3 PO 4 When the pH value is 2.12-7.2, phosphorus mainly exists in the form of H 2 PO 4 - When the pH value is 7.2-12.36, phosphorus mainly exists in the form of HPO 4 2- When the pH value is 12.36-14, phosphorus mainly exists in the form of PO 4 3- exists in the form of.

[0036] 2. After filtration, the concentration of Ca, Mg, Si and Al impurities is detected. Among them, Ca, Mg and Al are mainly Ca 2+ Mg 2+ 、Al 3+ There are ions in the form of SiO 2 Form precipitation, Mg 2+ The impurity content is high;

[0037] 3. Add EDTA to remove Mg in the solution 2+ ; Add LiOH to adjust the pH to 12.5-13 and heat (a small amount of monovalent metal ions, excess oxalate, PO 4 - And a small amount of HPO 4 2- ), where Li 3 PO 4 It exists in the form of a precipitate; the filtrate and the NH generated during the heating process 3 Enter H 3 PO 4 (NH 4 ) 2 HPO 4 ;

[0038] 4. Detection of Li 3 PO 4 C in wet material 2 O 4 2- Mg 2+ , where C 2 O 4 2-The impurity content is high, adding H 3 PO 4 and H 2 O 2 , to obtain lithium dihydrogen phosphate, where H 2 C 2 O 4 +H 2 O 2 =2CO 2 +2H 2 O;

[0039] 5. Evaporation, cooling, centrifugation and drying.

[0040] The waste lithium molecular sieves used in the following examples are all adsorbents discarded from the pressure swing adsorption oxygen generator of Yingde Gas Group, and the lithium content thereof is 4.15 wt%.

[0041] Example 1

[0042] A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0043] 1. Grind 200g of waste lithium molecular sieve through a 100-mesh sieve and mix with 894.4g of 35wt% diammonium hydrogen phosphate solution, where n(NH 4 + ):n(Li + )=4:1, immediately add oxalic acid solid, adjust the slurry pH=6.0, stir for 2h, and filter to obtain filter residue and filtrate;

[0044] 2. The residue was filtered again in step 1, and then mixed with the filtrate in step 1 to obtain solution 1. The concentrations of Ca, Mg, Si, and Al impurities in solution 1 were 5.1×10 -3 g / L, 0.1g / L, 3×10 -3 g / L、6×10 -4 g / L;

[0045] 3. Add EDTA (ethylenediaminetetraacetic acid) to solution 1 to satisfy n(EDTA):n(Mg 2+ )=1.5:1, after stirring for 15 minutes, add 7.2g of lithium hydroxide monohydrate, adjust the pH to 13 and heat to boiling, boil for 5 minutes and filter while hot, pass the filtrate after filtration and the ammonia volatilized during the evaporation process into 546.8g of 85wt% phosphoric acid to form a diammonium hydrogen phosphate solution, which is evenly divided for step 1 and step 2;

[0046] 4. After filtering, lithium phosphate wet material is obtained and Mg in the wet material is detected. 2+ , C 2 O 4 2-The impurity contents were 3.2ppm and 257ppm respectively, and then pure water was added to obtain a slurry, and the slurry was added to 85wt% phosphoric acid and 30wt% hydrogen peroxide to meet the requirements of Li 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:2.1, and the mass ratio of H to pure water is 1:1.85. 2 O 2 ):n(C 2 O 4 2- )=5:1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0047] 5. Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 125°C, cool to 35°C, then centrifuge and dry to obtain 140.1g of battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0048] Lithium yield = lithium output / lithium input * 100% = (140.1g / 104 * 99.82%) / ((8.3g / 7) + (7.2g / 42)) * 100%, lithium input includes lithium in lithium molecular sieve and lithium in lithium hydroxide monohydrate, that is, the molar ratio of lithium. The following examples and comparative examples are calculated in the same way and will not be repeated here.

[0049] Example 2

[0050] A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0051] 1. Grind 200g of waste lithium molecular sieve through a 100-mesh sieve and mix with 894.4g of 30.6% diammonium hydrogen phosphate solution, where n(NH 4 + ):n(Li + )=3.5:1, immediately add oxalic acid solid, adjust the slurry pH=5.8, stir for 3h, and filter to obtain filter residue and filtrate;

[0052] 2. The filter residue was filtered through the above step 1 and mixed with the filtrate in step 1 to obtain solution 1. The concentrations of Ca, Mg, Si and Al impurities in solution 1 were 5.3×10 -3 g / L, 0.19 g / L, 2.6×10 -3 g / L、8.9×10 -4 g / L;

[0053] 3. Add EDTA to solution 1 to satisfy n(EDTA):n(Mg 2+)=1:1, after stirring for 10 minutes, add 2.3g of lithium hydroxide monohydrate, adjust the pH to 12.5 and heat to boiling, boil for 5 minutes and filter while hot, pass the filtrate after filtration and the ammonia volatilized during the evaporation process into 478.5g of 85% phosphoric acid to form a diammonium hydrogen phosphate solution, which is evenly divided for step 1 and step 2;

[0054] 4. After filtering, lithium phosphate wet material is obtained and Mg in the wet material is detected. 2+ , C 2 O 4 2- The impurity contents were 22.7ppm and 285ppm respectively, and then pure water was added to obtain a slurry, and the slurry was added to 85% phosphoric acid and 30% hydrogen peroxide to meet the requirements of Li in the lithium phosphate wet material. 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:2.1 and the mass ratio of H to pure water is 1:1.9. 2 O 2 ):n(C 2 O 4 2- )=4:1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0055] 5. Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 130°C, cool to 40°C, then centrifuge and dry to obtain 126.1g of battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0056] Example 3

[0057] A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0058] 1. Grind 200g of waste lithium molecular sieve through a 100-mesh sieve and mix with 894.4g of 39.4% diammonium hydrogen phosphate solution, where n(NH 4 + ):n(Li + )=4.5:1, immediately add oxalic acid solid, adjust the slurry pH to 5, stir for 3h, and filter to obtain a filter residue and a filtrate;

[0059] 2. The residue was filtered again in step 1 and mixed with the filtrate in step 1 to obtain solution 1. The concentrations of Ca, Mg, Si and Al impurities in solution 1 were 3.1×10 -3 g / L, 0.07 g / L, 2.1×10 -2 g / L、9.3×10 -3 g / L;

[0060] 3. Add EDTA to solution 1 to satisfy n(EDTA):n(Mg 2+ )=1.3:1, after stirring for 15 minutes, add 6.9g of lithium hydroxide monohydrate, adjust the pH to 13 and heat to boiling, boil for 5 minutes and filter while hot, pass 615.2g of 85% phosphoric acid into the filtrate after filtration and the ammonia volatilized during the evaporation process to form a diammonium hydrogen phosphate solution, which is evenly divided for step 1 and step 2;

[0061] 4. After filtering, lithium phosphate wet material is obtained and Mg in the wet material is detected. 2+ , C 2 O 4 2- The impurity contents were 2.6ppm and 657ppm respectively, and then pure water was added to obtain a slurry, and the slurry was added to 85% phosphoric acid and 30% hydrogen peroxide to meet the requirements of Li in the lithium phosphate wet material. 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:2 and the mass ratio of H to pure water is 1:1.85. 2 O 2 ):n(C 2 O 4 2- )=3:1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0062] 5. Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 127°C, cool to 37°C, then centrifuge and dry to obtain 139.6g of battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0063] Comparative Example 1

[0064] A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0065] 1. Grind 200g of waste lithium molecular sieve through a 100-mesh sieve and mix with 1137.4g of 20.7% ammonium phosphate solution, where n(NH 4 + ):n(Li + )=4:1, immediately add oxalic acid solid, adjust the slurry pH=6.0, stir for 2h, and filter to obtain filter residue and filtrate;

[0066] 2. The residue was filtered again in step 1 and mixed with the filtrate in step 1 to obtain solution 1. The concentrations of Ca, Mg, Si and Al impurities in solution 1 were 2.7×10 -3 g / L, 0.06g / L, 5×10 -3 g / L、8×10-4 g / L;

[0067] 3. Add EDTA to solution 1 to satisfy n(EDTA):n(Mg 2+ )=1.5:1, after stirring for 15 minutes, add 8.9g of lithium hydroxide monohydrate, adjust the pH to 13 and heat to boiling, boil for 5 minutes and filter while hot, pass the filtrate after filtration and the ammonia volatilized during the evaporation process into 364.3g of 85% phosphoric acid to form an ammonium phosphate solution, which is evenly divided for step 1 and step 2;

[0068] 4. After filtering, lithium phosphate wet material is obtained and Mg in the wet material is detected. 2+ , C 2 O 4 2- The impurity contents were 2.9ppm and 7357ppm respectively, and then pure water was added to obtain a slurry, and the slurry was added to 85% phosphoric acid and 30% hydrogen peroxide to meet the requirements of Li in the lithium phosphate wet material. 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:2.1 and the mass ratio of H to pure water is 1:1.85. 2 O 2 ):n(C 2 O 4 2- )=5:1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0069] 5. Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 125°C, cool to 35°C, then centrifuge and dry to obtain 141.9g of battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0070] Comparative Example 1 Compared with Example 1, other conditions remain unchanged, and only ammonium phosphate is used to replace diammonium hydrogen phosphate. Since the ammonium phosphate solution is highly alkaline, the pH of the solution is generally between 11-12, and more oxalic acid is needed to adjust the pH to 6, so the obtained product oxalate is higher, and the insoluble matter is higher. In addition, since the solubility of ammonium phosphate is lower, it is 26.1g / 100mL, so more pure water is needed for preparation; if diammonium hydrogen phosphate is used to replace diammonium hydrogen phosphate, since the acidity of diammonium hydrogen phosphate solution is generally between 4.2-4.6, which is lower than 6, there is no need to add oxalic acid, and it is impossible to use oxalic acid to remove metal impurities, so it is not necessary to repeat them here. Therefore, only diammonium hydrogen phosphate can be used.

[0071] Comparative Example 2

[0072] A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves comprises the following steps:

[0073] 1. Grind 200g of waste lithium molecular sieve through a 100-mesh sieve and mix with 894.4g of 35% diammonium hydrogen phosphate solution, where n(NH 4 + ):n(Li + )=4:1, immediately add oxalic acid solid, adjust the slurry pH=6.0, stir for 2h, and filter to obtain filter residue and filtrate;

[0074] 2. The filter residue was then leached with 894.4 g of pure water, filtered and mixed with the filtrate in step 1 to obtain solution 1. The concentrations of Ca, Mg, Si, and Al impurities in solution 1 were detected to be 7.4×10 -3 g / L, 0.12g / L, 2×10 -3 g / L、8×10 -4 g / L;

[0075] 3. Add EDTA to solution 1 to satisfy n(EDTA):n(Mg 2+ )=1.5:1, after stirring for 15 minutes, add 7.1g of lithium hydroxide monohydrate, adjust the pH to 13 and heat to boiling, boil for 5 minutes and filter while hot, pass the filtrate and the ammonia volatilized during the evaporation process into 273.4g of 85% phosphoric acid to form a diammonium hydrogen phosphate solution, which is evenly divided for step 1;

[0076] 4. After filtering, lithium phosphate wet material is obtained and Mg in the wet material is detected. 2+ , C 2 O 4 2- The impurity contents were 3.5ppm and 176ppm respectively, and then pure water was added to obtain a slurry, and the slurry was added to 85% phosphoric acid and 30% hydrogen peroxide to meet the requirements of Li in the lithium phosphate wet material. 3 PO 4 With H in phosphoric acid solution 3 PO 4 The molar ratio of H is 1:2.1, and the mass ratio of H to pure water is 1:1.85. 2 O 2 ):n(C 2 O 4 2- )=5:1, after the slurry is dissolved, a pure lithium dihydrogen phosphate solution is obtained;

[0077] 5. Evaporate the pure lithium dihydrogen phosphate liquid, stop heating when the solution temperature reaches 125°C, cool to 35°C, then centrifuge and dry to obtain 132.0g of battery-grade lithium dihydrogen phosphate, and the centrifuged mother liquor enters step 4 to replace part of the pure water.

[0078] Compared with Example 1, Comparative Example 2 adopts one salt leaching and one water leaching. It can be seen that this method does not have a high lithium yield as the two salt leaching in Example 1.

[0079] Table 1 Content of various substances in battery-grade lithium dihydrogen phosphate

[0080]

[0081] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing battery-grade lithium dihydrogen phosphate using waste lithium molecular sieves, characterized in that: The following steps are involved: Step 1: Grind and sieve the waste lithium molecular sieve and mix it with diammonium phosphate solution, wherein the NH4 + and Li + The molar ratio of is (3.5-4.5):1, and then immediately adding oxalic acid solid, adjusting the slurry pH to 5-6, stirring for a preset time, and filtering to obtain a filter residue and a filtrate; Step 2: subjecting the filter residue to the operation of step 1 above, and mixing the filtrate obtained after filtration with the filtrate in step 1 to obtain solution 1; Step 3: According to the Mg in solution 1 2+ concentration, adding EDTA to the solution 1, stirring for a preset time, adding lithium hydroxide monohydrate, adjusting the pH value to 12.5-13, and then heating to boiling, and filtering while hot after boiling for a preset time to obtain lithium phosphate wet material; wherein, the EDTA and Mg 2+ The molar ratio is (1-1.5):1; Step 4: Detecting Mg in the lithium phosphate wet material 2+ and C2O4 2- The impurity content and the Li3PO4 content are then added into pure water to obtain a slurry, and the slurry is mixed with phosphoric acid and hydrogen peroxide, wherein the molar ratio of Li3PO4 in the lithium phosphate wet material to H3PO4 in the phosphoric acid solution is 1:(2-2.1), and the mass ratio of the lithium phosphate wet material to pure water is 1:(1.85-1.9), and the H2O2 and C2O4 in the hydrogen peroxide are 2- The molar ratio is (2-5):1, and after the slurry is dissolved, a lithium dihydrogen phosphate clean solution is obtained; Step 5: Heat the lithium dihydrogen phosphate clean solution to evaporate, stop heating when the solution temperature reaches 125-130° C., cool to a preset temperature, and then centrifuge and dry to obtain battery-grade lithium dihydrogen phosphate.

2. The method according to claim 1, characterized in that In step 1, the mass concentration of the diammonium hydrogen phosphate solution is 30-40%.

3. The method according to claim 1, characterized in that In step 1, the sieving is through a 100-mesh sieve; and the preset stirring time is 1-3 hours.

4. The method according to claim 1, characterized in that: In step 3, the preset stirring time is 10-15 minutes.

5. The method according to claim 1, characterized in that In step 3, the preset boiling time is 5-10 minutes.

6. The method according to claim 1, characterized in that In step 3, the filtered filtrate and the ammonia volatilized during the heating evaporation process are introduced into phosphoric acid to form a diammonium hydrogen phosphate solution which is evenly divided and returned to step 1 and step 2 for repeated use.

7. The method according to claim 3, characterized in that In step 3, the mass concentration of the phosphoric acid introduced is 83-88%.

8. The method according to claim 1, characterized in that In step 4, the mass concentration of the phosphoric acid is 83-88%; the mass concentration of the hydrogen peroxide is 28-33%.

9. The method according to claim 1, characterized in that: In step 5, the preset temperature is 35-40°C.

10. The method according to claim 1, characterized in that In step 5, the mother liquor obtained by centrifugation enters step 4 to replace part of the pure water for reuse.

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