Method for co-production of lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor

Abstract

The present application relates to resource recycling and wastewater treatment technical field, disclose a kind of method for lithium precipitation mother liquor coproduction lithium phosphate and ammonium magnesium phosphate.Utilize the method, which includes: (1) lithium precipitation mother liquor is separated by nanofiltration membrane, and nanofiltration permeate is obtained;(2) the nanofiltration permeate and phosphorus source are mixed, and the mixture obtained is adjusted to 11.5-13 by lye pH value, carries out first precipitation reaction, and obtains lithium phosphate and phosphorus-containing wastewater;(3) the phosphorus-containing wastewater, magnesium salt and ammonium salt are mixed, and the mixture obtained is adjusted to 9-10.5 by lye pH value, carries out second precipitation reaction, and obtains ammonium magnesium phosphate.The technical scheme realizes the comprehensive resource utilization and harmlessness of lithium precipitation mother liquor, avoids the scaling of lithium carbonate in membrane system, and guarantees the long-term stable operation of membrane system;And create a feasible reaction environment for ammonium magnesium phosphate precipitation method, solve the problem of failure in high carbonate wastewater due to competitive precipitation.

Description

Technical Field

[0001] This invention relates to the field of resource recycling and wastewater treatment technology, specifically to a method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor. Background Technology

[0002] Lithium carbonate is a key raw material in the lithium battery industry chain. During its production, a large amount of lithium-precipitated mother liquor is generated by adding sodium carbonate to a lithium-rich solution to induce a precipitation reaction. This mother liquor is a complex alkaline system with high sodium, high carbonate, and low lithium content, typically containing lithium ions, carbonate ions, and sodium ions. Approximately 25-35 m³ of this mother liquor is produced for every ton of lithium carbonate produced. While the lithium and carbonate resources contained within have significant recycling value, their direct discharge would result in resource waste and environmental pollution.

[0003] Currently, the industry mainly faces the following technical bottlenecks in the treatment and resource recovery of lithium precipitation mother liquor: (1) Incomplete resource recovery and low product value: Traditional processes such as "acidification decarbonization-evaporation concentration sodium precipitation method" can recover some lithium, but they have problems of high acid consumption and high energy consumption, and cannot recover carbonate resources; if the "phosphate precipitation method" is used to recover lithium, lithium phosphate can be obtained, but new phosphate pollution will be introduced, resulting in "lithium phosphate mother liquor" that is difficult to treat; thus, the present invention needs to solve two problems: first, recover sodium carbonate and lithium from the original mother liquor; second, treat the phosphorus-containing wastewater generated by its own process; (2) Scaling and Separation Challenges in Membrane Applications: Membrane separation technology, especially nanofiltration membranes, has shown great potential in lithium extraction from salt lakes and solution desalination due to its high efficiency and energy saving characteristics. Some patents propose using nanofiltration membrane desalination systems to recover carbonate ions from lithium precipitation mother liquor. However, the lithium precipitation mother liquor itself is a saturated or near-saturated solution of lithium carbonate. During nanofiltration concentration, it is very easy for lithium carbonate to precipitate out due to supersaturation, leading to severe membrane scaling and clogging, which affects the stable operation of the system. In addition, if phosphate ions are also present in the mother liquor, the concentrate produced by nanofiltration will be a mixed system of carbonate and phosphate, making it difficult to achieve high-value separation of the two. (3) Deep phosphorus removal technology fails in high carbonate systems: Magnesium ammonium phosphate precipitation is an effective technology to convert phosphorus in wastewater into slow-release fertilizer; however, this technology faces fundamental obstacles when treating lithium mother liquor or its derivative wastewater (such as lithium phosphate mother liquor); because the high concentration of carbonate in the solution will preferentially react with the added magnesium source to form magnesium carbonate precipitate, which seriously interferes with or even completely inhibits the formation of magnesium ammonium phosphate, resulting in low phosphorus removal efficiency and the inability to obtain pure fertilizer products; this makes the method unsuitable for direct application to this type of water quality. (4) Environmental pressure is becoming increasingly severe: With the tightening of environmental regulations, the emission limits for pollutants such as phosphorus in wastewater are becoming more and more stringent; the phosphorus content in the tail liquid of traditional processes often exceeds the standard, requiring additional investment in end-of-pipe treatment. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of existing technologies, such as the generation of difficult-to-handle "lithium phosphate mother liquor" during the treatment of lithium precipitation mother liquor, the fact that the mother liquor itself is a saturated or near-saturated solution of lithium carbonate, which easily leads to the precipitation of supersaturated lithium carbonate during nanofiltration concentration, resulting in severe membrane scaling and clogging, affecting the stable operation of the system, and the simultaneous presence of phosphate ions in the mother liquor, making it difficult to achieve high-value separation of carbonates and phosphates in the nanofiltration concentrate. This invention provides a method for co-producing lithium phosphate and magnesium ammonium phosphate from lithium precipitation mother liquor. This technical solution organically combines membrane separation with magnesium ammonium phosphate precipitation technology, breaking through technical bottlenecks in stages, achieving comprehensive resource utilization and harmlessness of lithium precipitation mother liquor, avoiding the precipitation and scaling of lithium carbonate in the membrane system, ensuring the long-term stable operation of the membrane system, and creating a feasible reaction environment for the magnesium ammonium phosphate precipitation method, solving the problem of the method failing due to competitive precipitation in high carbonate wastewater.

[0005] To achieve the above objectives, the present invention provides a method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor, the method comprising the following steps: (1) Separate the lithium precipitation mother liquor through a nanofiltration membrane to obtain nanofiltration permeate; (2) The nanofiltration permeate and phosphorus source are mixed, and the resulting mixture is subjected to pH adjustment to 11.5-13 with alkaline solution to carry out the first precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain lithium phosphate and phosphorus-containing wastewater. (3) The phosphorus-containing wastewater, magnesium salt and ammonium salt are mixed, and the resulting mixture is subjected to pH adjustment to 9-10.5 with alkaline solution to carry out a second precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain magnesium ammonium phosphate and tail liquid.

[0006] Preferably, in the lithium precipitation mother liquor, the content of carbonate ions is 25-40 g / L, the content of lithium ions is 1-3 g / L, the content of sodium ions is 40-50 g / L, and the content of phosphate ions is 5-10 g / L.

[0007] Preferably, in step (1), the separation conditions include: a pressure of 1.5-4 MPa and a temperature of 25-40°C.

[0008] Preferably, in step (2), the alkaline solution is an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide.

[0009] Preferably, in step (2), the conditions for the first precipitation include: a temperature of 40-90°C and a time of 30-120 min.

[0010] Preferably, step (2) further includes washing the lithium phosphate with water at a temperature of 50-60°C.

[0011] Preferably, in step (3), the magnesium salt is selected from at least one of magnesium chloride, magnesium sulfate, magnesium oxide and magnesium hydroxide.

[0012] Preferably, in step (3), the ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonia and ammonium phosphate.

[0013] Preferably, in step (3), the molar ratio of the phosphorus-containing wastewater, magnesium salt and ammonium salt is 1:(1-1.2):(1-1.2), wherein the phosphorus-containing wastewater is measured in molar amounts of phosphorus, the magnesium salt in molar amounts of magnesium, and the ammonium salt in molar amounts of nitrogen.

[0014] Preferably, in step (3), the conditions for the second precipitation include: a temperature of 20-40°C and a time of 20-60 min.

[0015] Compared with the prior art, the present invention has the following technical effects: (1) The method described in this invention adopts the process sequence of "pre-decarbonation membrane", which first uses nanofiltration membrane to selectively separate and recover sodium carbonate. This step not only realizes the resource utilization of carbonate, but more importantly, it completely eliminates the interference of high-concentration carbonate to the two subsequent core processes: first, it avoids the precipitation and scaling of lithium carbonate in the membrane system, ensuring the long-term stable operation of the membrane system; second, it creates a feasible reaction environment for the magnesium ammonium phosphate precipitation method, solving the problem that the method fails due to competitive precipitation in high carbonate wastewater. (2) The method described in this invention improves product quality and economic benefits: By removing impurities in the pre-process, a pure raw material liquid is provided for the preparation of lithium phosphate, which significantly reduces the content of sodium, a key impurity in the product, from more than 0.7% in the traditional process to less than 0.2%, which significantly improves the product grade and market value of lithium phosphate. At the same time, the process produces lithium phosphate and magnesium ammonium phosphate, which maximizes the recovery of resources and has significant economic benefits. (3) The method described in this invention achieves deep phosphorus removal and emission compliance: After removing carbonate interference, the magnesium ammonium phosphate method can operate efficiently, and can deeply treat the total phosphorus in the tail liquid to below 1 mg / L, while recovering it into fertilizer products with economic value, turning waste into treasure, and truly realizing the resource utilization of pollutants; (4) The process flow of the method described in this invention is compact and easy to implement and promote: the operation technology is mature, the conditions are mild, and it is easy to modify and integrate on the basis of existing lithium carbonate production equipment; the whole process has no secondary pollution and meets the requirements of green chemical industry and circular economy. (5) The method described in this invention has strong process flexibility and good resistance to water quality fluctuations: Through the core membrane separation unit, the concentration of key interference factor (carbonate) entering the subsequent process can be stably controlled at a low level, creating a stable operating window for the subsequent precipitation reaction; Even if the concentrations of lithium, carbonate and phosphate in the raw water of lithium precipitation mother liquor fluctuate within a certain range, by adjusting the operating pressure, recovery rate and other parameters of the nanofiltration system, as well as the amount of reagent added in the subsequent process, the stability of the final product quality and effluent indicators can be guaranteed, thus enhancing the industrial applicability of the process; (6) The method described in this invention is cost-effective and has prominent green circular characteristics: This process directly reuses sodium carbonate, saving the raw material consumption of the main process; it produces high-purity lithium phosphate and magnesium ammonium phosphate fertilizer, creating significant economic benefits; compared with the traditional "acidification-evaporation" or single precipitation method, although it increases the investment in membrane system, it significantly reduces acid consumption, energy consumption and end-of-pipe treatment costs, and has better long-term economic performance; at the same time, the entire process converts waste into three products, ultimately achieving near-zero wastewater discharge, which is in line with the principles of green chemistry and circular economy. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of the method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor as described in this invention. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0019] like Figure 1 As shown, the method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor according to the present invention includes the following steps: (1) Separate the lithium precipitation mother liquor through a nanofiltration membrane to obtain nanofiltration permeate; (2) The nanofiltration permeate and phosphorus source are mixed, and the resulting mixture is subjected to pH adjustment to 11.5-13 with alkaline solution to carry out the first precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain lithium phosphate and phosphorus-containing wastewater. (3) The phosphorus-containing wastewater, magnesium salt and ammonium salt are mixed, and the resulting mixture is subjected to pH adjustment to 9-10.5 with alkaline solution to carry out a second precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain magnesium ammonium phosphate and tail liquid.

[0020] The method described in this invention avoids the precipitation and scaling of lithium carbonate in the membrane system, ensuring the long-term stable operation of the membrane system. Simultaneously, it creates a feasible reaction environment for the magnesium ammonium phosphate precipitation method, solving the problem of its failure due to competitive precipitation in high-carbonate wastewater. Furthermore, by pre-removing impurities, a pure feedstock solution is provided for the preparation of lithium phosphate, significantly reducing the sodium content, a key impurity in the product, from over 0.7% in traditional processes to below 0.2%, thereby significantly improving the product grade and market value of lithium phosphate.

[0021] In the method described in this invention, the content of carbonate ions in the lithium precipitation mother liquor can be 25-40 g / L, preferably 28.5-32 g / L; the content of lithium ions can be 1-3 g / L, preferably 1.8-2.2 g / L; the content of sodium ions can be 40-50 g / L, preferably 45-50 g / L; and the content of phosphate ions can be 5-10 g / L, preferably 8.5-10 g / L.

[0022] In the method described in this invention, the separation conditions in step (1) include: the pressure can be 1.5-4 MPa, preferably 2.5-3.2 MPa; the temperature can be 25-40℃, preferably 30-35℃. In this document, the pressure is gauge pressure.

[0023] In the method described in this invention, in step (1), the nanofiltration membrane can be an alkali-resistant nanofiltration membrane. The alkali-resistant nanofiltration membrane is preferably a piperazine amide type or a polyamide composite membrane with a special surface treatment. The alkali-resistant nanofiltration membrane should have a rejection rate of greater than 95% for divalent carbonate ions and a rejection rate of less than 20% for monovalent lithium ions. To optimize carbonate recovery rate and operating energy consumption, the nanofiltration membrane preferably adopts a single-stage or multi-stage design.

[0024] In some embodiments, the nanofiltration membrane includes a first-stage nanofiltration membrane and a second-stage nanofiltration membrane, which are connected in series. The pressure of the first-stage nanofiltration membrane is 1.5-3 MPa, and the pressure of the second-stage nanofiltration membrane is 3-4 MPa.

[0025] In the method described in this invention, in step (1), the lithium precipitation mother liquor is separated by a nanofiltration membrane to obtain a nanofiltration concentrate. The nanofiltration membrane has a high rejection rate for high-valence ions and can effectively enrich carbonate and sodium ions in the mother liquor on the concentration side to form a sodium carbonate solution with a concentration of 80-150 g / L, which can be directly reused in the upstream lithium carbonate precipitation process. Lithium ions mainly enter the permeate side. In step (1), by controlling the concentration ratio or adopting a multi-stage series design, the carbonate concentration in the permeate can be stably controlled below 2.0 g / L, and ideally reduced to below 1.0 g / L, to completely eliminate its interference with subsequent steps.

[0026] In the method described in this invention, step (1) may further include: reusing the nanofiltration concentrate in the lithium carbonate precipitation process. Step (1) may further include: evaporating and crystallizing the nanofiltration concentrate to obtain sodium carbonate solid. Step (1) may further include: monitoring the concentration of the nanofiltration concentrate using an online conductivity meter. The value of the recovered sodium carbonate can cover most of the operating power consumption and reagent costs of the membrane system, while the sale of lithium phosphate and magnesium ammonium phosphate is pure profit, indicating good economic benefits according to preliminary estimates.

[0027] In the method described in this invention, in step (2), in order to ensure complete lithium precipitation and avoid excessive phosphorus, the molar ratio of the amount of nanofiltration permeate to the amount of phosphorus source is preferably (2.5-3.5):1, more preferably (3-3.5):1; wherein, the nanofiltration permeate is in the molar amount of lithium, and the phosphorus source is in the molar amount of phosphate.

[0028] In the method described in this invention, in step (2), the alkaline solution can be an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide, preferably an aqueous solution of sodium hydroxide. The concentration of the alkaline solution can be 5-30 wt%, preferably 10-20 wt%.

[0029] In the method described in this invention, in step (2), the obtained mixture is subjected to pH adjustment by alkaline solution, and the pH value can be 11.5-13, preferably 11.5-12.2.

[0030] In the method described in this invention, in step (2), to ensure complete lithium precipitation, the conditions for the first precipitation include: a temperature preferably of 40-90°C, more preferably 50-85°C; and a time preferably of 30-120 min, more preferably 70-90 min. Since the membrane separation in step (1) has removed a large amount of sodium ions and competing carbonate ions, the reaction environment is pure, and therefore the purity of the obtained lithium phosphate product is significantly improved.

[0031] In the method described in this invention, in step (2), the solid-liquid separation can be performed by filtration.

[0032] In the method described in this invention, step (2) may further include: washing the lithium phosphate with water, wherein the water temperature is 50-60°C. Step (2) may further include: drying the washed lithium phosphate. The drying conditions include: a temperature of 80-120°C, preferably 100-110°C; and a time of 60-180 min, preferably 120-150 min.

[0033] In the method described in this invention, in step (3), the magnesium salt may be selected from at least one of magnesium chloride, magnesium sulfate, magnesium oxide and magnesium hydroxide, preferably magnesium chloride.

[0034] In the method described in this invention, in step (3), the ammonium salt may be selected from at least one of ammonium chloride, ammonium sulfate, ammonia and ammonium phosphate, preferably ammonium chloride.

[0035] In the method described in this invention, in step (3), the molar ratio of the amount of phosphorus-containing wastewater, magnesium salt and ammonium salt can be 1:(1-1.2):(1-1.2), preferably 1:(1-1.1):(1-1.1), wherein the phosphorus-containing wastewater is measured in molar amounts of phosphorus, the magnesium salt is measured in molar amounts of magnesium, and the ammonium salt is measured in molar amounts of nitrogen.

[0036] In the method described in this invention, in step (3), the alkaline solution can be an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide, preferably an aqueous solution of sodium hydroxide. The concentration of the alkaline solution can be 5-30 wt%, preferably 10-20 wt%.

[0037] In the method described in this invention, in step (3), the obtained mixture is subjected to pH adjustment by alkaline solution, and the pH value can be 9-10.5, preferably 9.5-10.

[0038] In the method described in this invention, in step (3), the conditions for the second precipitation include: the temperature can be 20-40℃, preferably 25-35℃; the time can be 20-60min, preferably 40-60min.

[0039] In the method described in this invention, in step (3), the solid-liquid separation can be performed by filtration.

[0040] In the method described in this invention, most of the additional sodium ions introduced by the alkali added in step (2) will remain in the tail liquid in step (3). Step (3) may also include: discharging the tail liquid. Step (3) may also include: using the tail liquid for water replenishment. Step (3) may also include: when the concentration of sodium ions in the tail liquid is below 10 g / L, using the tail liquid to rinse the nanofiltration membrane, and returning the rinsed nanofiltration membrane to step (1) for repeated membrane separation; when the concentration of sodium ions in the tail liquid is greater than 10 g / L, evaporating and crystallizing the tail liquid, and managing the resulting mixed salts as solid waste.

[0041] In some implementations, such as Figure 1 As shown, the method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor according to the present invention includes the following steps: (1) Separate the lithium precipitation mother liquor through a nanofiltration membrane at a pressure of 1.5-4 MPa and a temperature of 25-40℃ to obtain nanofiltration permeate; (2) The nanofiltration permeate and phosphorus source are mixed, and the pH of the resulting mixture is adjusted to 11.5-13 by alkaline solution. The mixture is then subjected to precipitation reaction at 40-90℃ for 30-120 min. The resulting mixture is then separated into solid and liquid by filtration to obtain lithium phosphate and phosphorus-containing wastewater. The lithium phosphate is washed with water at 50-60℃ and then dried at 80-120℃ for 60-180 min. (3) Mix the phosphorus-containing wastewater, magnesium salt and ammonium salt, and adjust the pH of the mixture to 9-10.5 with alkaline solution. Then carry out a precipitation reaction at 20-40℃ for 20-60 min. Next, separate the solid and liquid components of the mixture by filtration to obtain magnesium ammonium phosphate and tail liquid. Use the tail liquid to rinse the nanofiltration membrane, and return the rinsed nanofiltration membrane to step (1) for repeated membrane separation. In the lithium precipitation mother liquor, the content of carbonate ions is 25-40 g / L, the content of lithium ions is 1-3 g / L, the content of sodium ions is 40-50 g / L, and the content of phosphate ions is 5-10 g / L; the molar ratio of the amount of nanofiltration permeate to the amount of phosphorus source is (2.5-3.5):1, wherein the amount of nanofiltration permeate is based on the molar amount of lithium, and the amount of phosphorus source is based on the molar amount of phosphate; the molar ratio of the amount of phosphorus-containing wastewater, magnesium salt, and ammonium salt is 1:(1-1.2):(1-1.2), wherein the amount of phosphorus-containing wastewater is based on the molar amount of phosphorus, the amount of magnesium salt is based on the molar amount of magnesium, and the amount of ammonium salt is based on the molar amount of nitrogen.

[0042] In other implementations, such as Figure 1 As shown, the method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor according to the present invention includes the following steps: (1) Separate the lithium precipitation mother liquor through a nanofiltration membrane at a temperature of 25-40℃ to obtain nanofiltration permeate, wherein the nanofiltration membrane includes a first-stage nanofiltration membrane and a second-stage nanofiltration membrane, and the first-stage nanofiltration membrane and the second-stage nanofiltration membrane are connected in series, wherein the pressure of the first-stage nanofiltration membrane is 1.5-3MPa, and the pressure of the second-stage nanofiltration membrane is 3-4MPa; (2) Mix the nanofiltration permeate and the phosphorus source, and adjust the pH of the mixture to 11.5-13 with alkaline solution. Perform a precipitation reaction at 50-80℃ for 70-90 min. Then, separate the solid and liquid components of the mixture by filtration to obtain lithium phosphate and phosphorus-containing wastewater. Wash the lithium phosphate with water at 50-60℃ and then dry it at 100-110℃ for 120-150 min. (3) Mix the phosphorus-containing wastewater, magnesium salt and ammonium salt, and adjust the pH of the mixture to 9-10.5 with alkaline solution. Then carry out the precipitation reaction at 25-35℃ for 40-60 min. Next, filter the mixture to separate solid and liquid, and obtain magnesium ammonium phosphate and tail liquid. Use the tail liquid to rinse the nanofiltration membrane, and return the rinsed nanofiltration membrane to step (1) for repeated membrane separation. In the lithium precipitation mother liquor, the carbonate ion content is 28.5-32 g / L, the lithium ion content is 1.8-2.2 g / L, the sodium ion content is 40-50 g / L, and the phosphate content is 8.5-10 g / L; the molar ratio of the nanofiltration permeate to the phosphorus source is (3-3.5):1, wherein the nanofiltration permeate is expressed in molar amounts of lithium, and the phosphorus source is expressed in molar amounts of phosphate; in step (2), the alkaline solution is... The alkaline solution is an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide, with a concentration of 5-30 wt%; the molar ratio of the phosphorus-containing wastewater, magnesium salt and ammonium salt is 1:(1-1.1):(1-1.1), wherein the phosphorus-containing wastewater is measured in molar amounts of phosphorus, the magnesium salt in molar amounts of magnesium, and the ammonium salt in molar amounts of nitrogen; in step (3), the alkaline solution is an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide, with a concentration of 5-30 wt%.

[0043] The following examples further illustrate the method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor according to the present invention. These examples are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following examples.

[0044] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following embodiments are commercially available.

[0045] Example 1 (1) 1000L of lithium precipitation mother liquor (in which the content of carbonate ions is 28.5g / L, the content of lithium ions is 1.8g / L, the content of sodium ions is 45g / L, and the content of phosphate ions is 8.5g / L) is preheated to 35°C and pumped into a single-stage alkali-resistant nanofiltration membrane. The operating pressure is set to 3.0MPa. After the operation is stable, the concentration of the nanofiltration concentrate is monitored by an online conductivity meter, and the system recovery rate is controlled at 85% to ensure that the carbonate concentration in the obtained nanofiltration permeate is stable at 1.2g / L (the volume of the nanofiltration permeate is 850L, of which CO32- is 1000g / L). 2- When the concentration drops below 1.2 g / L, Li + (effective retention was achieved), and 150L of nanofiltration concentrate was collected, in which the Na2CO3 concentration reached 112g / L and the carbonate recovery rate was 96%; the obtained nanofiltration concentrate was evaporated and crystallized to obtain 17kg of sodium carbonate solid (calculated as Na2CO3). (2) Phosphoric acid (the molar ratio of the nanofiltration permeate to the phosphorus source is 3.05:1, wherein the nanofiltration permeate is in the molar amount of lithium and the phosphorus source is in the molar amount of phosphate) is added to the nanofiltration permeate and mixed. The resulting mixture is then subjected to pH adjustment to 12.2 by passing it through a 20wt% sodium hydroxide aqueous solution. A precipitation reaction is carried out at 80°C for 70 min. The resulting mixture is then subjected to solid-liquid separation by pressure filtration to obtain lithium phosphate wet filter cake and phosphorus-containing wastewater. The lithium phosphate is washed with water at 55°C and then dried at 105°C for 120 min to obtain 3.1 kg of lithium phosphate product. The product has a main content (Li3PO4) of 98.1% and a sodium (Na) content of 0.16%. (3) The phosphorus-containing wastewater obtained in step (2) is transferred to a reaction vessel, and industrial-grade magnesium chloride hexahydrate and ammonium chloride are added (the molar ratio of the phosphorus-containing wastewater, magnesium chloride and ammonium chloride is 1:1:1, wherein the phosphorus-containing wastewater is calculated by the molar amount of phosphorus, magnesium chloride by the molar amount of magnesium, and ammonium chloride by the molar amount of nitrogen). The resulting mixture is then subjected to pH adjustment to 9.5 by passing it through a 20wt% sodium hydroxide aqueous solution. A precipitation reaction is carried out at 30℃ for 40 min. The resulting mixture is then allowed to stand and subjected to solid-liquid separation by filtration to obtain wet-based magnesium ammonium phosphate filter cake (4.0 kg on a dry basis, with a precipitation efficiency of 98.5%) and tailings. The tailings are tested, and the results are: Li + With a concentration of 0.15 g / L and a total phosphorus (TP) concentration of <1 mg / L, the effluent quality consistently meets national discharge standards.

[0046] Example 2 (1) 1000L of lithium precipitation mother liquor (in which the content of carbonate ions is 30.0g / L, the content of lithium ions is 2.0g / L, the content of sodium ions is 48.2g / L, and the content of phosphate ions is 9.0g / L) is preheated to 30°C, and a two-stage alkali-resistant nanofiltration membrane is pumped in (the nanofiltration membrane includes a first-stage nanofiltration membrane and a second-stage nanofiltration membrane, and the first-stage nanofiltration membrane and the second-stage nanofiltration membrane are connected in series). The pressure of the first-stage nanofiltration membrane is 2.5MPa, and the pressure of the second-stage nanofiltration membrane is 3.2MPa. After the operation is stable, the concentration of the concentrate is monitored by an online conductivity meter, and the system recovery rate is controlled at 88% to ensure that the carbonate concentration in the obtained nanofiltration permeate is stable at 0.8g / L (the volume of the nanofiltration permeate is 880L, of which CO32- is 1000g / L). 2- When the concentration drops below 0.8 g / L, Li + (effective retention was achieved); and 120 L of nanofiltration concentrate was collected, in which the Na2CO3 concentration reached 125 g / L and the carbonate recovery rate was 96.8%; the obtained nanofiltration concentrate was evaporated and crystallized to obtain 15.0 kg of sodium carbonate solid (calculated as Na2CO3). (2) Phosphoric acid (the molar ratio of the nanofiltration permeate to the phosphorus source is 3.0:1, wherein the nanofiltration permeate is in the molar amount of lithium and the phosphorus source is in the molar amount of phosphate) is added to the nanofiltration permeate and mixed. The resulting mixture is then subjected to pH adjustment to 11.5 by passing it through a 20wt% sodium hydroxide aqueous solution. A precipitation reaction is carried out at 40°C for 120 min. The resulting mixture is then subjected to solid-liquid separation by pressure filtration to obtain lithium phosphate wet filter cake and phosphorus-containing wastewater. The lithium phosphate is washed with water at 55°C and then dried at 105°C for 120 min to obtain 3.4 kg of lithium phosphate product. The product has a main content (Li3PO4) of 98.5% and a sodium (Na) content of 0.18%. (3) The phosphorus-containing wastewater obtained in step (2) is transferred to a reaction vessel, and industrial-grade magnesium chloride hexahydrate and ammonium chloride are added (the molar ratio of the phosphorus-containing wastewater, magnesium chloride and ammonium chloride is 1:1.1:1.1, wherein the phosphorus-containing wastewater is calculated in molar amounts of phosphorus, magnesium chloride in molar amounts of magnesium, and ammonium chloride in molar amounts of nitrogen). The resulting mixture is then subjected to pH adjustment to 9.0 with a 20wt% sodium hydroxide aqueous solution, and a precipitation reaction is carried out at 25°C for 60 min. The resulting mixture is then allowed to stand and subjected to solid-liquid separation by filtration to obtain wet-based magnesium ammonium phosphate filter cake (4.3 kg on a dry basis, with a precipitation efficiency of 99.0%) and tailings. The tailings are tested, and the results are: Li + The concentration is 0.18 g / L, and the total phosphorus (TP) concentration is <1 mg / L. The effluent quality consistently meets the national discharge standards.

[0047] Example 3 (1) 1000L of lithium precipitation mother liquor (in which the content of carbonate ions is 32.0g / L, the content of lithium ions is 2.2g / L, the content of sodium ions is 50.0g / L, and the content of phosphate ions is 10.0g / L) is preheated to 40℃ and pumped into a two-stage alkali-resistant nanofiltration membrane (the nanofiltration membrane includes a first-stage nanofiltration membrane and a second-stage nanofiltration membrane, and the first-stage nanofiltration membrane and the second-stage nanofiltration membrane are connected in series). The pressure of the first-stage nanofiltration membrane is 1.5MPa, and the pressure of the second-stage nanofiltration membrane is 4MPa. After the operation is stable, the concentration of the concentrate is monitored by an online conductivity meter, and the control system recovery rate is controlled at 82% to ensure that the carbonate concentration in the obtained nanofiltration permeate is stable at 0.5g / L (the volume of the nanofiltration permeate is 820L, of which CO32-200L is 1000g / L). 2- When the concentration drops below 0.5 g / L, Li + (effective retention was achieved), and 180L of nanofiltration concentrate was collected, in which the Na2CO3 concentration reached 142g / L and the carbonate recovery rate was 96.8%; the obtained nanofiltration concentrate was evaporated and crystallized to obtain 25.6kg of sodium carbonate solid (calculated as Na2CO3). (2) Phosphoric acid (the molar ratio of the nanofiltration permeate to the phosphorus source is 3.5:1, wherein the nanofiltration permeate is in the molar amount of lithium and the phosphorus source is in the molar amount of phosphate) is added to the nanofiltration permeate and mixed. The resulting mixture is then subjected to pH adjustment to 13.0 by passing it through a 15wt% sodium hydroxide aqueous solution. A precipitation reaction is carried out at 85°C for 30 min. The resulting mixture is then subjected to solid-liquid separation by pressure filtration to obtain lithium phosphate wet filter cake and phosphorus-containing wastewater. The lithium phosphate is washed with water at 55°C and then dried at 110°C for 120 min to obtain 3.8 kg of lithium phosphate product. The product has a main content (Li3PO4) of 98.8% and a sodium (Na) content of 0.15%. (3) The phosphorus-containing wastewater obtained in step (2) is transferred to a reaction vessel, and industrial-grade magnesium chloride hexahydrate and ammonium chloride are added (the molar ratio of the phosphorus-containing wastewater, magnesium chloride and ammonium chloride is 1:1.1:1.1, wherein the phosphorus-containing wastewater is calculated in molar amounts of phosphorus, magnesium chloride in molar amounts of magnesium, and ammonium chloride in molar amounts of nitrogen). The resulting mixture is then subjected to pH adjustment to 10.0 using a 15wt% sodium hydroxide aqueous solution, and a precipitation reaction is carried out at 35°C for 20 minutes. The resulting mixture is then allowed to stand and subjected to solid-liquid separation by filtration to obtain wet-based magnesium ammonium phosphate filter cake (4.5 kg on a dry basis, with a precipitation efficiency of 99.5%) and tailings. The tailings are tested, and the results are: Li + With a concentration of 0.20 g / L and a total phosphorus (TP) concentration of <1 mg / L, the effluent quality consistently meets national discharge standards.

[0048] Comparative Example 1 The method of Example 1 is implemented, except that step (1) is omitted, as follows: (1) Phosphoric acid (the molar ratio of the lithium precipitation mother liquor to the phosphoric acid source is 3.05:1, wherein the lithium precipitation mother liquor is calculated as the molar amount of lithium carbonate ions is 28.5 g / L, the lithium ion content is 1.8 g / L, the sodium ion content is 45 g / L, and the phosphate content is 8.5 g / L) is added to 1000 L of lithium precipitation mother liquor (the content of lithium precipitation mother liquor is calculated as the molar amount of lithium, and the phosphoric acid source is calculated as the molar amount of phosphate) and mixed. The resulting mixture is then subjected to pH adjustment to 12.2 by passing a 20 wt% sodium hydroxide aqueous solution. The precipitation reaction is carried out at 80 °C for 70 min. The resulting mixture is then subjected to solid-liquid separation by pressure filtration to obtain lithium phosphate wet filter cake and phosphorus-containing wastewater. The lithium phosphate is washed with water at 55 °C and then dried at 105 °C for 120 min to obtain 3.0 kg of lithium phosphate product. The product has a main content (Li3PO4) of 95.2% and a sodium (Na) content of 0.75%. (2) The phosphorus-containing wastewater obtained in step (1) is transferred to a reaction vessel, and industrial-grade magnesium chloride hexahydrate and ammonium chloride are added (the molar ratio of the phosphorus-containing wastewater, magnesium chloride and ammonium chloride is 1:1:1, wherein the phosphorus-containing wastewater is calculated by the molar amount of phosphorus, magnesium chloride by the molar amount of magnesium, and ammonium chloride by the molar amount of nitrogen). The resulting mixture is then subjected to pH adjustment to 9.5 with a 20wt% sodium hydroxide aqueous solution, and a precipitation reaction is carried out at 30℃ for 40 min. The resulting mixture is then allowed to stand and subjected to solid-liquid separation by filtration to obtain wet-based magnesium ammonium phosphate filter cake (2.5 kg on a dry basis, with a precipitation efficiency of 65%) and tailings. The tailings are tested, and the results are: Li + The concentration was 0.18 g / L, and the total phosphorus (TP) concentration was 85 mg / L.

[0049] The results from the examples and comparative examples show that the lithium phosphate product prepared by the method of using lithium precipitation mother liquor to co-produce lithium phosphate and magnesium ammonium phosphate described in this invention has high purity, high precipitation efficiency of magnesium ammonium phosphate, and the total phosphorus in the tail liquor is deeply treated to below 1 mg / L.

[0050] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for co-producing lithium phosphate and magnesium ammonium phosphate using lithium precipitation mother liquor, characterized in that, The method includes the following steps: (1) Separate the lithium precipitation mother liquor through a nanofiltration membrane to obtain nanofiltration permeate; (2) The nanofiltration permeate and phosphorus source are mixed, and the resulting mixture is subjected to pH adjustment to 11.5-13 with alkaline solution to carry out the first precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain lithium phosphate and phosphorus-containing wastewater. (3) The phosphorus-containing wastewater, magnesium salt and ammonium salt are mixed, and the resulting mixture is subjected to pH adjustment to 9-10.5 with alkaline solution to carry out a second precipitation reaction. Then the resulting mixture is subjected to solid-liquid separation to obtain magnesium ammonium phosphate and tail liquid.

2. The method according to claim 1, characterized in that, In the lithium precipitation mother liquor, the content of carbonate ions is 25-40 g / L, the content of lithium ions is 1-3 g / L, the content of sodium ions is 40-50 g / L, and the content of phosphate ions is 5-10 g / L.

3. The method according to claim 1 or 2, characterized in that, In step (1), the separation conditions include: pressure of 1.5-4 MPa and temperature of 25-40℃.

4. The method according to any one of claims 1-3, characterized in that, In step (2), the alkaline solution is an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide.

5. The method according to any one of claims 1-4, characterized in that, In step (2), the conditions for the first precipitation include: a temperature of 40-90℃ and a time of 30-120min.

6. The method according to any one of claims 1-5, characterized in that, Step (2) further includes washing the lithium phosphate with water at a temperature of 50-60°C.

7. The method according to any one of claims 1-6, characterized in that, In step (3), the magnesium salt is selected from at least one of magnesium chloride, magnesium sulfate, magnesium oxide and magnesium hydroxide.

8. The method according to any one of claims 1-7, characterized in that, In step (3), the ammonium salt is selected from at least one of ammonium chloride, ammonium sulfate, ammonia and ammonium phosphate.

9. The method according to any one of claims 1-8, characterized in that, In step (3), the molar ratio of the phosphorus-containing wastewater, magnesium salt and ammonium salt is 1:(1-1.2):(1-1.2), wherein the phosphorus-containing wastewater is measured in molar amounts of phosphorus, the magnesium salt in molar amounts of magnesium, and the ammonium salt in molar amounts of nitrogen.

10. The method according to any one of claims 1-9, characterized in that, In step (3), the conditions for the second precipitation include: a temperature of 20-40°C and a time of 20-60 min.

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