Method for preparing alkali metal hydrophosphate from slag obtained after lithium extraction of invalid lithium iron phosphate

Abstract

The invention provides a method for preparing alkali metal hydrophosphate from slag after lithium extraction of failed lithium iron phosphate, which comprises the following steps: reacting alkali metal orthophosphate with the slag after lithium extraction of failed lithium iron phosphate to obtain high-purity alkali metal hydrophosphate; on the basis, reaction paths and parameters are optimized, efficient recovery of phosphorus resources and iron resources in the slag obtained after lithium extraction is achieved, the technological process is simple, and the alkali consumption is small.

Description

Technical Field

[0001] The present invention relates to the field of recycling of lithium iron phosphate, and particularly to a method for preparing alkali metal hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate. Background Art

[0002] With the rapid development of the new energy vehicle industry, the power batteries of new energy vehicles promoted in the early stage in China have successively entered the end-of-life period, and a large number of retired batteries will face the problem of recycling. At present, the proportion of lithium iron phosphate batteries in retired lithium batteries is gradually increasing. After the retired batteries are crushed, powdered, and flotation, the spent lithium iron phosphate cathode material can be obtained. The mainstream treatment process of the spent lithium iron phosphate cathode material is the wet process, that is, through sulfuric acid-hydrogen peroxide oxidation leaching, the lithium in the cathode is extracted, and then lithium carbonate is prepared. After the spent lithium iron phosphate is leached for lithium by the wet process, the residue mainly composed of iron phosphate - the residue after lithium extraction is left. The residue after lithium extraction has complex components, high impurity content, and contains a large amount of phosphorus and iron elements. It can be both a resource and pollute the environment. Therefore, the resource-based and harmless treatment of the residue after lithium extraction requires a new way out.

[0003] At present, in the recycling of the residue after lithium extraction from waste lithium iron phosphate, there is a process of acid leaching and precipitation to prepare iron phosphate, but the products obtained by this process are difficult to meet the performance requirements of the precursor for synthesizing lithium iron phosphate. In addition to the above process, there are other problems in the recycling of the residue after lithium extraction from waste lithium iron phosphate in the existing other processes: for example, in the invention patent CN116588909A, the phosphorus-iron slag is subjected to reduction acid leaching, impurities are removed by adjusting the pH, and then iron phosphate is acidified and oxidized and precipitated. The reducing agent used in this process cannot be reused, the impurity removal effect by only adjusting the pH is not good, and the loss of valuable elements is large, and the environmental protection pressure is high; in the invention patent CN119306259A, the residue after lithium extraction is first subjected to acid leaching treatment, and then an extractant is used for extraction to obtain a ferric chloride solution, but the process flow is long, and the production cost and environmental protection cost are high; in the invention patent CN118619224A, the residue after lithium extraction from waste lithium iron phosphate is slurried with water, and the pH is adjusted with dilute alkali solution, and phosphate is precipitated and crystallized. This process has a high alkali consumption and a limited profit space.

[0004] Aiming at the above process defects, the present invention proposes a method for preparing monohydrogen phosphate from the residue after lithium extraction from waste lithium iron phosphate. This method realizes the efficient recovery of phosphorus resources and iron resources in the residue after lithium extraction through an alkali solution leaching process, and has a simple process flow, less auxiliary material consumption, and a large profit space. Summary of the Invention

[0005] To solve the technical problems in the background art, the present invention proposes a method for preparing alkali metal hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate, which includes the following steps:

[0006] S1. React the alkali metal orthophosphate with the residue after lithium extraction from spent lithium iron phosphate, and after solid-liquid separation, obtain alkali metal hydrogen phosphate and alkali metal hydroxy iron phosphate residue.

[0007] In the prior art, alkali metal hydroxides are often used to extract phosphorus from the residue after lithium extraction. However, this extraction method not only has a high process cost and a large consumption of alkali, but also the final product contains a large amount of sodium phosphate. In the present invention, the alkali metal orthophosphate is innovatively reacted with the residue after lithium extraction, which not only has a low alkali consumption, but also the obtained product has a high purity (99.5% alkali metal hydrogen phosphate).

[0008] The reaction involved in step S1 is as follows:

[0009] Na 3 PO 4 +FePO 4 +H 2 O→NaFePO 4 (OH)+Na 2 HPO 4 (1)

[0010] K 3 PO 4 +FePO 4 +H 2 O→KFePO 4 (OH)+K 2 HPO 4 (2)

[0011] In the prior art, when using alkali metal hydroxides to extract phosphorus from the residue after lithium extraction, the reactions involved are as follows:

[0012] 2 NaOH+FePO 4 +H 2 O→Fe(OH) 3 +Na 2 HPO 4 (3)

[0013] 2 KOH+FePO 4 +H 2 O→Fe(OH) 3 +K 2 HPO 4 (4)

[0014] 3 NaOH+FePO 4 →Fe(OH) 3 +Na 3 PO 4 (5)

[0015] 3 KOH+FePO 4 →Fe(OH)3 +K 3 PO 4 (6)

[0016] The reaction involved in the present invention has more advantages compared with the prior art. This is because, from the perspective of reaction thermodynamics, reaction (3) or (4) is difficult to occur due to the influence of side reaction (5) or (6); moreover, the Gibbs free energy of reaction (5) or (6) is more negative and the heat release is greater. Therefore, even if an alkali with a phosphorus substance amount twice that in the reaction formula of reaction (3) or (4) is added, the reaction (i.e., (5) or (6)) to produce orthophosphate will still occur preferentially; and thereafter, since the amount of alkali added is insufficient to support the conversion of orthophosphate, the excess iron phosphate will react with the generated Na 3 PO4 or K 3 PO 4 to continue the reaction to produce a small amount of Na 2 HPO 4 or K 2 HPO 4 , but this process is very slow, resulting in an extremely low apparent phosphorus leaching rate.

[0017] A method for preparing alkali metal hydrogen phosphate from the slag after lithium extraction from waste lithium iron phosphate also includes the following steps:

[0018] S2. After recycling the alkali metal hydroxyphosphate iron slag in step S1, it is subjected to a solution reaction with an alkali metal hydroxide. After solid-liquid separation, alkali metal orthophosphate is obtained, and the alkali metal orthophosphate is recycled to step S1.

[0019] In the present invention, when the recycled alkali metal hydroxyphosphate iron slag reacts with an alkali metal hydroxide in solution, the solid-state phosphorus in the alkali metal hydroxyphosphate iron slag can be completely extracted into the solution, and the comprehensive phosphorus extraction rate can reach 99.5%; the reaction formula involved in step S2 is as follows:

[0020] Na x FePO 4 (OH) y +OH - →Fe(OH) 3 +Na 3 PO 4 (7)

[0021] K x FePO 4 (OH) y +OH - →Fe(OH) 3 +K 3 PO 4 (8)

[0022] In step (7) or (8), a strong alkaline environment provided by an alkali metal hydroxide is used to completely extract the solid phosphorus in the alkali metal hydroxyphosphate iron slag. This is mainly because OH - can accelerate the conversion of FePO 4 precipitate to Fe(OH) 3 precipitate, thereby quickly releasing phosphorus into the solution;

[0023] In the present invention, the raw material for step S2 in the subsequent rounds is the alkali metal hydroxyphosphate iron slag and the alkali metal hydroxide. In the initial startup stage, the raw materials used are the slag after lithium extraction from the spent lithium iron phosphate and the alkali metal hydroxide. The reactions involved in the initial round are as follows:

[0024] FePO 4 +3OH - ==Fe(OH) 3 +PO 4 3- (9)

[0025] Specifically, for the startup stage where the slag after lithium extraction needs to be input in the initial round, the dosage of the alkali metal hydroxide is 3.0 - 3.3 times the amount of phosphorus substance in the slag after lithium extraction.

[0026] In step S2, the solution reaction of the recovered alkali metal hydroxyphosphate iron slag with the alkali metal hydroxide includes the first phosphorus extraction and the second phosphorus extraction. The alkali metal hydroxide is potassium hydroxide or sodium hydroxide.

[0027] In the present invention, since the alkali concentration in the later stage of the reaction will decrease, it is difficult for the reaction to proceed further to the right, resulting in insufficient phosphorus leaching rate and ultimately reducing the product yield of phosphorus. And if we want to further leach out all the phosphorus, it often leads to the alkali consumption far exceeding the theoretical value. In order to achieve the purpose of deeply extracting phosphorus and reducing alkali consumption at the same time, a two-stage batchwise alkali addition method is adopted.

[0028] In step S2, the first phosphorus extraction includes the following steps: The alkali metal hydroxyphosphate iron slag recovered in step S1 is slurried to obtain slurry 1, and the alkali metal hydroxide is added to slurry 1 for reaction to obtain iron-containing filter residue 1 and alkali metal orthophosphate;

[0029] Preferably, the liquid-solid ratio of slurry 1 is 4 - 8:1;

[0030] Preferably, the molar ratio of the alkali metal hydroxide to the phosphorus contained in the alkali metal hydroxyphosphate iron slag is 1.6 - 2.0:1;

[0031] Preferably, the reaction temperature for the first phosphorus extraction is 60 - 70 °C, and the reaction time is 2 - 3 h.

[0032] In the iron-containing filter residue 1 obtained from the first phosphorus extraction, there is still some residual phosphorus, which affects the final resource utilization of the iron slag.

[0033] In step S2, the second phosphorus extraction includes the following steps: Pulverize the iron-containing filter residue 1 to obtain a slurry 2, add an alkali metal hydroxide to the slurry 2 and stir and wash. After solid-liquid separation, a filter residue and washing water are obtained. After the obtained filter residue is rinsed, an iron-containing filter residue 2 and rinsing water are obtained. The washing water and the rinsing water are mixed to obtain washing water 1.

[0034] In step S2, the iron-containing filter residue 2 is pulverized with pure water to obtain a slurry 2, and the liquid-solid ratio of the slurry 2 is 2:1; the molar ratio of the alkali metal hydroxide to the phosphorus contained in the alkali metal hydroxyphosphate iron salt slag is 0.2-0.6:1; the stirring and washing temperature for the second phosphorus extraction is 60-70 °C, and the stirring and washing time is 30-90 min; the liquid-solid ratio for rinsing during the second phosphorus extraction is 0.5-0.75:1, and the number of rinsing times is 2-3 times; the pH of the washing water 1 is 11.5-13.0.

[0035] In step S2, the recovery of the alkali metal hydroxyphosphate iron salt slag includes rinsing the alkali metal hydroxyphosphate iron salt slag with a rinsing liquid to obtain washing water 2; the pH of the rinsing liquid is 11.5-13.0;

[0036] Preferably, the rinsing liquid is an aqueous solution of an alkaline metal oxide, and more preferably the washing water 1 in step S2.

[0037] In step S2, there is alkalinity in the washing water 1 that has not been fully utilized, and a large amount of alkali metal hydrogen orthophosphate remains in the alkali metal hydroxyphosphate iron salt slag obtained in step S1. Therefore, the present invention preferably adopts a washing method, that is, using the washing water 1 obtained in step S2 for the recovery of the alkali metal hydroxyphosphate iron salt slag, converting the alkali metal hydrogen phosphate in the alkali metal hydroxyphosphate iron salt slag into alkali metal orthophosphate, achieving the dual goals of improving the comprehensive utilization rate of alkali and reducing water consumption.

[0038] In step S1, the temperature for the solution reaction of the alkali metal orthophosphate and the slag after lithium extraction from the failed lithium iron phosphate is 85-95 °C, the reaction time is 2-5 h, and the pH at the end of the reaction is 8.8-9.2;

[0039] Preferably, after the recovery of the alkali metal orthophosphate in step S2, it is subjected to a solution reaction with the slag after lithium extraction from the failed lithium iron phosphate;

[0040] More preferably, when the alkali metal orthophosphate is subjected to a solution reaction with the slag after lithium extraction from the failed lithium iron phosphate, the reaction system further includes the washing water 2 obtained during the recovery of the alkali metal hydroxyphosphate iron salt slag in step S2.

[0041] In the present invention, the alkali metal orthophosphate that undergoes a solution reaction with the residue after lithium extraction can be sodium phosphate or potassium phosphate purchased externally, or the alkali metal orthophosphate recovered in step S2; when step S1 recycles the alkali metal orthophosphate produced in step S2 and step S2 recycles the alkali metal hydroxyphosphate iron slag produced in step S1, step S1 and step S2 form a phosphorus extraction cycle. This cyclic process enables the phosphorus in the residue after lithium extraction to be efficiently extracted in the form of hydrogen phosphate.

[0042] A method for preparing alkali metal hydrogen phosphate from the residue after lithium extraction of spent lithium iron phosphate further includes:

[0043] S3. Using a defluorinating agent to remove fluorine from the alkali metal hydrogen phosphate in step S1 to obtain an alkali metal hydrogen phosphate product solution;

[0044] Preferably, the defluorinating agent is one of calcium chloride, calcium oxide, or calcium hydroxide;

[0045] Preferably, the molar ratio of the defluorinating agent to fluorine in the alkali metal hydrogen phosphate is 4 - 5:1.

[0046] There is usually a certain amount of fluorine remaining in the residue after lithium extraction of spent lithium iron phosphate (mainly from the electrolyte and binder). This fluorine will enter the solution during the above reaction, ultimately resulting in too high an F content in the product. In the present invention, the fluorine impurities are removed by the precipitation method.

[0047] It further includes:

[0048] S4. Inducing crystallization of the alkali metal hydrogen phosphate product solution in step S3 to obtain alkali metal hydrogen phosphate finished products and crystallization mother liquor; the seed crystal used for the induced crystallization is Na 2 HPO 4 ·12H 2 O or K 2 HPO 4 ·12H 2 O, and the amount of the seed crystal is 1 - 3‰ of the alkali metal hydrogen phosphate product solution;

[0049] Preferably, the alkali metal hydrogen phosphate product solution is cooled to 20 - 25 °C before induced crystallization, and the crystallization is terminated when the temperature drops to 10 °C;

[0050] Preferably, the crystallization mother liquor is used for the pulping in the first phosphorus extraction in step S2.

[0051] Advantages of the present invention:

[0052] (1) Through the solution reaction of the alkali metal orthophosphate with the residue after lithium extraction of spent lithium iron phosphate, the present invention obtains high-purity alkali metal hydrogen phosphate;

[0053] (2) By optimizing the leaching reaction path and reaction parameters, the present invention achieves the effect of deep phosphorus extraction while producing alkali metal hydrogen phosphates, effectively reducing the comprehensive alkali consumption and water consumption of the process, thereby reducing the overall cost of the process, providing a new feasible method for the battery recycling industry to achieve comprehensive recycling of all elements, greatly reducing the auxiliary material cost, especially the consumption of alkali, and thus greatly increasing the profit margin of the product;

[0054] (3) The alkali metal hydrogen phosphates produced by the present invention have greater market value and are widely used in fields such as chemical production, food, medicine, water treatment, and chemical fertilizers. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is a flow chart for preparing alkali metal hydrogen phosphates from the slag after lithium extraction from spent lithium iron phosphate in Examples 1-3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below in conjunction with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0057] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0058] Below, the technical solutions of the present invention will be described more clearly and completely in conjunction with specific embodiments and comparative examples.

[0059] Example 1

[0060] This example presents a method for preparing disodium hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate, and the specific steps are as follows:

[0061] S1. Take 3500 mL of 35 g P / L sodium orthophosphate solution obtained in the previous step S2, add 1280 g of wet slag after lithium extraction (containing 37% water and 18% phosphorus), react at 90 °C for 2 h and then filter to obtain an alkali metal hydrogen phosphate solution (sodium hydrogen phosphate solution) and filter residue;

[0062] S2. Directly rinse the filter residue obtained in step S1 with the washing water 1 obtained in the previous step S2, consume and convert the hydrogen phosphate contained in the filter residue into phosphate, and finally obtain an alkali metal hydroxyphosphate iron salt slag and washing water 2;

[0063] Take 1800 g of alkali metal hydroxyphosphate iron salt slag; make up the crystallization mother liquor in the previous step S4 with water to 4 L as the reaction solution, and use the reaction solution to slurry the alkali metal hydroxyphosphate iron salt slag to obtain slurry 1; then, according to the molar ratio of phosphorus to alkali of 1:2 in the slag after lithium extraction, weigh 380 g of industrial-grade flake caustic soda (NaOH), dissolve the flake caustic soda in the above slurry, and react at 70 °C and pH = 11.2 for 3 h. This reaction is the first phosphorus extraction, and filter to obtain iron-containing filter residue 1 and sodium orthophosphate solution; then, according to the molar ratio of phosphorus to alkali of 1:0.2 in the slag after lithium extraction, weigh 40 g of industrial-grade flake caustic soda (NaOH) and add it to 1200 mL of water, slurry the iron-containing filter residue 1 to obtain slurry 2, and stir and react the slurry 2 at 70 °C for 1 h for the second phosphorus extraction to obtain a filter cake and washing water; wash the filter cake obtained after the second phosphorus extraction with 250 mL of pure water twice to obtain iron-containing filter residue 2 and washing water, and mix the washing water and the washing water to obtain washing water 1;

[0064] In step S2, the elemental composition of the sodium orthophosphate solution is as follows: Li 110 ppm, K 1559 ppm, Ca 13.43 ppm, Mg 0.68 ppm, Fe 1147.8 ppm, Al 151.8 ppm, P 35407 ppm, F 121 ppm, pH 11.46;

[0065] The elemental composition of washing water 1 is as follows: Li 10 ppm, K 777.8 ppm, Ca 1.45 ppm, Mg 0.22 ppm, Fe 35.49 ppm, Al 36.8 ppm, P 21628 ppm, pH 11.70;

[0066] The elemental composition of iron-containing filter residue 2 is as follows: Li 1291.8 ppm, K 227.3 ppm, Ca 1844.4 ppm, Mg 112.4 ppm, Fe 354988 ppm, Al 842.5 ppm, P 3350 ppm, F 56 ppm.

[0067] The elemental composition of the alkali metal hydrogen phosphate solution (sodium hydrogen phosphate solution) is as follows: Li 10 ppm, K 407 ppm, Ca 1.04 ppm, Mg 0 ppm, Fe 95.69 ppm, Al 0.8 ppm, P 31203 ppm, F 198 ppm, pH 8.98;

[0068] The elemental composition of washing water 2 is as follows: Li 69.9 ppm, K 1533.4 ppm, Ca 5.5 ppm, Mg 0.12 ppm, Fe 774.6 ppm, Al 17.1 ppm, P 26320 ppm, F 69 ppm, pH 10.10.

[0069] After calculation, the comprehensive phosphorus extraction rate in step S2 is as high as 99.5%.

[0070] S3. According to the molar ratio of fluorine to calcium of 1:4, add calcium oxide to the sodium hydrogen phosphate solution obtained in step S1 to remove fluorine, and filter to obtain the sodium hydrogen phosphate product solution. At this time, the F content in the product solution is reduced to 14 ppm;

[0071] S4. Cool the sodium hydrogen phosphate product solution obtained in step S3 at a rate of 5 °C / h. When it is cooled to 22 °C, add 2‰ by mass of disodium hydrogen phosphate dodecahydrate as a seed for induced crystallization, continue to cool to 10 °C until crystallization is complete, filter, and air-dry at room temperature for 10 h to obtain disodium hydrogen phosphate dodecahydrate;

[0072] The composition of this product is as follows: disodium hydrogen phosphate dodecahydrate 99.56%, K 7 ppm, Ca 0 ppm, Mg 0 ppm, Fe 9.2 ppm, sulfate radical 1356 ppm, chlorine 13 ppm, F 78 ppm, pH = 9.11, and the appearance is pure white, meeting the national standard for industrial grade requirements.

[0073] Example 2

[0074] This example provides a method for preparing disodium hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate, and the specific steps are as follows:

[0075] This example provides a method for preparing disodium hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate, and the specific steps are as follows:

[0076] S1. Take 3200 mL of 35 g P / L sodium orthophosphate solution and 1050 mL of 26 g P / L washing water 2 obtained in the previous round of step S2, add 960 g of wet residue after lithium extraction (containing 37% water and 18% phosphorus), react at 90 °C for 2 h and then filter to obtain an alkali metal hydrogen phosphate solution (sodium hydrogen phosphate solution) and filter residue;

[0077] S2. Directly wash the filter residue obtained in step S1 with washing water 1 obtained in the previous round of step S2, consume and convert the hydrogen phosphate contained in the filter residue into phosphate radical, and finally obtain an alkali metal hydroxyphosphate iron salt residue and washing water 2;

[0078] Take 1350 g of alkali metal hydroxyphosphate iron slag; make up the crystallization mother liquor in the previous step S4 with water to 4 L as the reaction solution, and use the reaction solution to slurry the alkali metal hydroxyphosphate iron slag to obtain slurry 1; then, according to the molar ratio of phosphorus to alkali in the slag after lithium extraction of 1:1.8, weigh 256.5 g of industrial-grade flake caustic soda (NaOH), dissolve the flake caustic soda in the above slurry, and react at 70 °C and pH = 11.2 for 3 h. This reaction is the first phosphorus extraction, and filter to obtain iron-containing filter residue 1 and sodium orthophosphate solution; then, according to the molar ratio of phosphorus to alkali in the slag after lithium extraction of 1:0.5, weigh 75 g of industrial-grade flake caustic soda (NaOH) and add it to 900 mL of water, slurry the iron-containing filter residue 1 to obtain slurry 2, and stir and react the slurry 2 at 65 °C for 1 h for the second phosphorus extraction to obtain a filter cake and washing water; wash the filter cake obtained after the second phosphorus extraction 3 times with 300 mL of pure water to obtain iron-containing filter residue 2 and washing water, and mix the washing water and the washing water to obtain washing water 1;

[0079] In step S2, the elemental composition of the sodium orthophosphate solution is as follows: Li 105 ppm, K 1530 ppm, Ca 13.15 ppm, Mg 0.62 ppm, Fe 1149.8 ppm, Al 153.8 ppm, P 35527 ppm, F 116 ppm, pH 11.34;

[0080] The elemental composition of washing water 1 is as follows: Li 10 ppm, K 769.7 ppm, Ca 1.42 ppm, Mg 0.23 ppm, Fe 35.39 ppm, Al 39.4 ppm, P 21523 ppm, pH 11.61;

[0081] The elemental composition of iron-containing filter residue 2 is as follows: Li 1281.9 ppm, K 225.2 ppm, Ca 1857.3 ppm, Mg 113.6 ppm, Fe 354875 ppm, Al 843.7 ppm, P 3372 ppm, F 57 ppm.

[0082] The elemental composition of the alkali metal hydrogen phosphate solution (sodium hydrogen phosphate solution) is as follows: Li 12 ppm, K 412 ppm, Ca 1.06 ppm, Mg 0 ppm, Fe 95.52 ppm, Al 0.8 ppm, P 31108 ppm, F 185 ppm, pH 9.06;

[0083] The elemental composition of washing water 2 is as follows: Li 68.3 ppm, K 1532.6 ppm, Ca 5.36 ppm, Mg 0.13 ppm, Fe 763.4 ppm, Al 15.6 ppm, P 26457 ppm, F 67.1 ppm, pH 10.13.

[0084] After calculation, the comprehensive phosphorus extraction rate of step S2 is as high as 99.49%.

[0085] S3. According to the molar ratio of fluorine to calcium of 1:4, calcium oxide is added to the sodium hydrogen phosphate solution obtained in step S1 to remove fluorine, and then filtered to obtain a sodium hydrogen phosphate product solution. At this time, the F content in the product solution is reduced to 13 ppm.

[0086] S4. The sodium hydrogen phosphate product solution obtained in step S3 is cooled at a rate of 5 °C / h. When it is cooled to 22 °C, 3‰ by mass of disodium hydrogen phosphate dodecahydrate is added as a seed crystal to induce crystallization. Then it is continuously cooled to 10 °C to complete crystallization, filtered, and air-dried at room temperature for 10 h to obtain disodium hydrogen phosphate dodecahydrate.

[0087] The composition of this product is as follows: disodium hydrogen phosphate dodecahydrate 99.55%, K 8 ppm, Ca 0 ppm, Mg 0 ppm, Fe 8.7 ppm, sulfate radical 1295 ppm, chlorine 14 ppm, F 66 ppm, pH = 9.12, and the appearance is pure white, meeting the national standard for industrial grade requirements.

[0088] Example 3

[0089] This example proposes a method for preparing potassium hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate, and the specific steps are as follows:

[0090] S1. Take 3500 mL of 35 g P / L potassium orthophosphate solution obtained in the previous round of step S2, add 1280 g of wet slag after lithium extraction (containing 37% water and 18% phosphorus), react at 95 °C for 5 h, and then filter to obtain an alkali metal hydrogen phosphate solution (potassium hydrogen phosphate solution) and filter residue.

[0091] S2. The filter residue obtained in step S1 is directly leached with the washing water 1 obtained in the previous round of step S2, and the hydrogen phosphate radicals contained in the filter residue are consumed and converted into phosphate radicals, finally obtaining an alkali metal hydroxyphosphate slag (potassium hydroxyphosphate) and washing water 2.

[0092] Take 1800 g of alkali metal hydroxyphosphate iron slag; Make up the crystallization mother liquor in the previous step S4 with water to 4 L as the reaction solution, and use the reaction solution to slurry the alkali metal hydroxyphosphate iron slag to obtain slurry 1; Then, according to the molar ratio of phosphorus to alkali of 1:2 in the slag after lithium extraction, weigh 380 g of industrial-grade flake caustic soda (NaOH), dissolve the flake caustic soda in the above slurry, and react at 70 °C and pH = 11.2 for 3 h. This reaction is the first phosphorus extraction, and filter to obtain iron-containing filter residue 1 and sodium orthophosphate solution; Then, according to the molar ratio of phosphorus to alkali of 1:0.4 in the slag after lithium extraction, weigh 80 g of industrial-grade flake caustic soda (NaOH) and add it to 1200 mL of water, slurry the iron-containing filter residue 1 to obtain slurry 2, and stir and react the slurry 2 at 70 °C for 1 h for the second phosphorus extraction to obtain a filter cake and washing water; The filter cake obtained after the second phosphorus extraction is rinsed twice with 250 mL of pure water to obtain iron-containing filter residue 2 and rinsing water, and the washing water and the rinsing water are mixed to obtain washing water 1;

[0093] In step S2, the elemental composition of the potassium orthophosphate solution is as follows: Li 108 ppm, K 107780 ppm, Ca 12.57 ppm, Mg 0.57 ppm, Fe 1235.1 ppm, Al 162.3 ppm, P 35419 ppm, F 127 ppm, pH 11.39;

[0094] The elemental composition of washing water 1 is as follows: Li 11 ppm, K 65661.8 ppm, Ca 1.33 ppm, Mg 0.15 ppm, Fe 36.54 ppm, Al 38.2 ppm, P 21593 ppm, pH 11.63;

[0095] The elemental composition of iron-containing filter residue 2 is as follows: Li 1295.2 ppm, K 352.8 ppm, Ca 1955.4 ppm, Mg 126.7 ppm, Fe 355210 ppm, Al 837.5 ppm, P 3396 ppm, F 59 ppm.

[0096] The elemental composition of the alkali metal hydrogen phosphate solution (potassium hydrogen phosphate solution) is as follows: Li 6 ppm, K 62813 ppm, Ca 1.21 ppm, Mg 0 ppm, Fe 94.87 ppm, Al 0.76 ppm, P 31192 ppm, F 185 ppm, pH 8.86;

[0097] The elemental composition of washing water 2 is as follows: Li 69.5 ppm, K 54173.4 ppm, Ca 5.8 ppm, Mg 0.11 ppm, Fe 789.2 ppm, Al 17.1 ppm, P 26582 ppm, F 58 ppm, pH 10.07.

[0098] After calculation, the comprehensive phosphorus extraction rate in step S2 is as high as 99.45%.

[0099] S3. According to the molar ratio of fluorine to calcium of 1:4, calcium hydroxide is added to the sodium hydrogen phosphate solution obtained in step S1 to remove fluorine, and then filtered to obtain the sodium hydrogen phosphate product solution. At this time, the F content in the product solution is reduced to 11 ppm.

[0100] S4. The sodium hydrogen phosphate product solution obtained in step S3 is cooled at a rate of 5 °C / h. When it is cooled to 22 °C, 2‰ by mass of disodium hydrogen phosphate dodecahydrate is added as a seed for induced crystallization, and then it is continuously cooled to 10 °C to complete crystallization. After filtration and air drying at room temperature for 12 h, disodium hydrogen phosphate dodecahydrate is obtained.

[0101] The composition of this product is as follows: 99.56% of dipotassium hydrogen phosphate dodecahydrate, 0 ppm of Ca, 0 ppm of Mg, 8.5 ppm of Fe, 1295 ppm of sulfate radical, 13 ppm of chlorine, 83 ppm of F, pH = 9.02, and the appearance is pure white, meeting the national standard for industrial grade requirements.

[0102] Comparative Example 1

[0103] This comparative example presents a method for preparing disodium hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate. The specific steps are as follows: Steps S3 and S4 are the same as those in Example 1, except that steps S1 and S2 in Example 1 are equivalently replaced with "Take 1280 g of wet residue after lithium extraction (containing 37% water and 18% phosphorus), 380 g of industrial-grade caustic soda, and 5120 mL of pure water, react at 90 °C for 2 h and then filter to obtain a filtrate and a filter residue. Then, react the above filter residue, 40 g of industrial-grade caustic soda, and 1200 mL of pure water at 90 °C for 2 h and filter. Collect the filtrates from the two phosphorus extraction steps."

[0104] In Comparative Example 1, although the operations such as deep phosphorus extraction are the same as those in Example 1, directly adding alkali for the leaching of disodium hydrogen phosphate is thermodynamically affected by the competition of the reaction to form sodium phosphate, and the proportion of sodium phosphate in the leaching solution is extremely high (30%). When adding disodium hydrogen phosphate for induced crystallization subsequently, the quality of the obtained disodium hydrogen phosphate is much lower than that in Example 1. After calculation, the comprehensive phosphorus extraction rate in Comparative Example 1 is 52%.

[0105] Comparative Example 2

[0106] This comparative example presents a method for preparing disodium hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate. The specific steps are the same as those in Example 1, except that in step S1, "react at 90 °C for 2 h and then filter" is changed to "react at 80 °C for 2 h and then filter".

[0107] In Comparative Example 2, the color of the disodium hydrogen phosphate leaching solution was extremely deep. After testing, the Fe content reached 1103 ppm. After impurity removal and crystallization of this solution, the product obtained was as follows: the main content was 99.06%, K was 14 ppm, Ca was 0 ppm, Mg was 0 ppm, Fe was 598.2 ppm, sulfate was 1428 ppm, chlorine was 22 ppm, P was 31203 ppm, F was 64 ppm, pH was 9.21, and the appearance was light yellow. This indicates that too low a reaction temperature is not conducive to the destabilization of the iron hydroxide colloid, resulting in an increase in the iron content of the disodium hydrogen phosphate leaching solution, ultimately making the iron content of the product unqualified, with poor whiteness and affecting the product quality.

[0108] As described above, only the preferred specific embodiments of the present invention are provided, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. A method for preparing alkali metal hydrogen phosphate from spent lithium iron phosphate residue, characterized in that: The steps include: S1. The alkali metal orthophosphate is reacted with the spent lithium iron phosphate residue after lithium extraction, and after solid-liquid separation, an alkali metal hydrogen phosphate and an alkali metal hydroxy iron phosphate residue are obtained.

2. The method for preparing alkali metal hydrogen phosphate from spent lithium iron phosphate residue after lithium extraction according to claim 1, characterized in that: The following steps are also included: S2. Recover the alkali metal hydroxyferric phosphate slag in step S1 and react it with alkali metal hydroxide to obtain alkali metal orthophosphate after solid-liquid separation. The alkali metal orthophosphate is recovered to step S1.

3. The method for preparing alkali metal hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate according to claim 1 or 2, characterized in that: In step S2, the alkali metal hydroxyferric phosphate slag is recovered and then subjected to a solution reaction with an alkali metal hydroxide, including a first phosphorus extraction and a second phosphorus extraction, and the alkali metal hydroxide is potassium hydroxide or sodium hydroxide.

4. The method for preparing alkali metal hydrogen phosphate from spent lithium iron phosphate residue after lithium extraction according to claim 3, characterized in that: In step S2, the first phosphorus extraction comprises the following steps: slurrying the alkali metal hydroxyferric phosphate slag recovered in step S1 to obtain slurry 1, adding alkali metal hydroxide to the slurry 1 to react, and obtaining iron-containing filter residue 1 and alkali metal orthophosphate; Preferably, the liquid-to-solid ratio of the slurry 1 is 4-8:1; Preferably, the molar ratio of the alkali metal hydroxide to the phosphorus contained in the alkali metal hydroxyferric phosphate slag is 1.6-2.0:1; Preferably, the reaction temperature of the first phosphorus extraction is 60-70°C, and the reaction time is 2-3h.

5. The method for preparing alkali metal hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate according to claim 3, characterized in that: In step S2, the second phosphorus extraction includes the following steps: slurrying the iron-containing filter residue 1 to obtain slurry 2, adding alkali metal hydroxide to the slurry 2 for stirring and washing, and obtaining filter residue and stirring and washing water after solid-liquid separation, eluting the obtained filter residue to obtain iron-containing filter residue 2 and eluting water, and the stirring and washing water are mixed with the eluting water to obtain washing water 1.

6. The method for preparing alkali metal hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate according to claim 5, characterized in that: In step S2, the iron-containing filter residue 2 is slurried with pure water to obtain slurry 2, and the liquid-to-solid ratio of the slurry 2 is 2:1; the molar ratio of the alkali metal hydroxide to the phosphorus contained in the alkali metal hydroxyferric phosphate slag is 0.2-0.6:1; the stirring and washing temperature of the second phosphorus extraction is 60-70°C, and the stirring and washing time is 30-90min; the liquid-to-solid ratio of the elution during the second phosphorus extraction is 0.5-0.75:1, and the number of elutions is 2-3 times; the pH of the wash water 1 is 11.5-13.

0.

7. The method for preparing alkali metal hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate according to claim 5 or 6, characterized in that: In step S2, the recovery of the alkali metal hydroxy ferric phosphate slag includes eluting the alkali metal hydroxy ferric phosphate slag with an eluent to obtain wash water 2; the pH of the eluent is 11.5-13.0; Preferably, the eluent is an aqueous solution of alkaline metal oxides, and more preferably is the wash water 1 of step S2.

8. The method for preparing alkali metal hydrogen phosphate from the residue after lithium extraction from spent lithium iron phosphate according to claim 7, characterized in that: In step S1, the temperature of the solution reaction between the alkali metal orthophosphate and the spent lithium iron phosphate residue after lithium extraction is 85-95° C., the reaction time is 2-5 hours, and the pH at the end point of the reaction is 8.8-9.2; Preferably, in step S2, the alkali metal orthophosphate is recovered and reacted with the spent lithium iron phosphate residue after lithium extraction; Further preferably, when the alkali metal orthophosphate and the spent lithium iron phosphate residue after lithium extraction are subjected to a solution reaction, the reaction system also includes the wash water 2 obtained when the alkali metal hydroxyferric phosphate residue is recovered in step S2.

9. The method for preparing alkali metal hydrogen phosphate according to any one of claims 1 to 8, characterized in that: Also includes: S3, using a defluorinating agent to defluorinate the alkali metal hydrogen phosphate in step S1 to obtain an alkali metal hydrogen phosphate product liquid; Preferably, the defluoridating agent is one of calcium chloride, calcium oxide or calcium hydroxide; Preferably, the molar ratio of the defluorinating agent to the fluorine in the alkali metal hydrogen phosphate is 4-5:

1.

10. The method for preparing alkali metal hydrogen phosphate from the slag after lithium extraction from spent lithium iron phosphate according to any one of claims 4 to 9, characterized in that: Also includes: S4, inducing crystallization of the alkali metal hydrogen phosphate product solution in step S3 to obtain an alkali metal hydrogen phosphate product and a crystallization mother liquor; the seed crystal used for the induced crystallization is Na2HPO4·12H2O or K2HPO4·12H2O, and the amount of the seed crystal is 1-3‰ of the hydrogen phosphate product solution; Preferably, the hydrogen phosphate product liquid is cooled to 20-25°C and then crystallization is induced, and crystallization is terminated when the temperature drops to 10°C; Preferably, the crystallization mother liquor is used for slurrying for the first phosphorus extraction in step S2.

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