Method for selectively recovering lithium from low-concentration lithium-containing solution

Abstract

The invention discloses a method for selectively recovering lithium from a low-concentration lithium-containing solution, which comprises the following steps: 1) adjusting the pH value of lithium-containing wastewater to be alkaline, filtering, adjusting the pH value of the lithium-containing wastewater to be 0.2-0.8 g / L, and adjusting the contents of Na and K in the lithium-containing wastewater to be 1-10 g / L; 2) adding an organic carboxylic acid extraction-precipitation agent into the filtrate, heating to a certain temperature, and stirring for reaction; 3) cooling and filtering to obtain filtrate and a filter cake; 4) washing the filter cake, and adding inorganic acid for back extraction to strip lithium loaded in the filter cake, so as to obtain a regenerated extraction-precipitant and a concentrated lithium-containing solution; and 5) adjusting the concentrated lithium-containing solution to be alkaline, deeply removing impurities and filtering, and preparing the battery-grade lithium carbonate from the filtrate through back-adding precipitation. According to the method, lithium is extracted from low-concentration resin regeneration wastewater based on an organic carboxylic acid extraction-precipitation method, lithium can be directly extracted from a low-concentration lithium-containing solution, and meanwhile concentration and extraction-precipitator circulation of lithium can be achieved through organic acid stripping.

Description

Technical Field

[0001] The invention relates to the technical field of lithium extraction from low-concentration lithium-containing wastewater, and in particular to a method for selectively recovering lithium from a low-concentration lithium-containing solution. Background Art

[0002] In recent years, with the increasing popularity of electric vehicles and the growing demand for electrochemical energy storage, the demand for lithium has surged. Li2CO3, LiOH and LiCl are the three main lithium salts used in industry, which are essential for the manufacture of lithium products such as power lithium-ion batteries (LIBs) and energy storage lithium-ion batteries (LIBs). Among these lithium salts, Li2CO3 is the most important, accounting for more than 50% of the market. Li2CO3 is divided into industrial grade, battery grade and high-purity grade products according to purity. According to statistics, battery-grade Li2CO3 accounts for more than 80% of the total consumption. Since the solubility of Li2CO3 in 100g of water is greater than 0.8g, 4 types of wastewater will be generated in the process of producing Li2CO3, namely resin regeneration wastewater, Li2CO3 precipitation mother liquor, Li2CO3 washing wastewater and carbonization-decomposition wastewater. Battery-grade Li2CO3 production companies usually combine these four types of wastewater as Li2CO3 precipitation mother liquor for unified treatment.

[0003] Generally, the pH value of Li2CO3 precipitation mother liquor is adjusted to neutral first, and then the lithium content is concentrated to more than 15g / L. The concentrated Li2CO3 precipitation mother liquor can be used to prepare industrial-grade Li2CO3. The current lithium recovery process not only requires repeated addition of acid and alkali to adjust the pH value, but also causes significant lithium loss due to the sodium salt precipitation process. In response to these problems, a variety of methods have been proposed to selectively extract lithium from Li2CO3 precipitation mother liquor, including chemical precipitation, extraction, adsorption and electrochemical adsorption. The precipitation mother liquor after lithium extraction can be used to recover sodium by evaporation crystallization or frozen crystallization. Compared with the other three lithium-containing wastewaters, the concentrations of lithium (0.2-0.8g / L) and sodium (1-2g / L) in resin regeneration wastewater are much lower, which makes the combined treatment of resin regeneration wastewater and Li2CO3 precipitation mother liquor less efficient and more energy-intensive. Therefore, the development of efficient and sustainable technology for extracting lithium from resin regeneration wastewater is crucial to the development of the lithium industry.

[0004] The extraction of lithium from Li2CO3 precipitation mother liquor and high sodium brine has received widespread attention. Annunzia et al. used sodium phosphate precipitation to extract lithium from saturated sodium chloride brine. After precipitation, the lithium concentration in the brine can be reduced to 40ppm, and the lithium recovery rate can reach 84%. Due to the relatively small market size of lithium phosphate, the subsequent preparation of Li2CO3 or LiOH still requires Li / PO4 3- Separation and PO4 3-recovery, which limits its large-scale application in the field of lithium recovery. In addition to Na3PO4 precipitation, NaF precipitation can be used to recover lithium from LPML. When the lithium and sodium concentrations were 5.6 g / L and 112.6 g / L, respectively, the recovery rate of lithium in LPML was 91.3%. Subsequently, LiF was converted into LiOH solution using Ca(OH)2. Unfortunately, since the solubility of LiF in water is significantly higher than that of Li3PO4, the lithium content in the aqueous phase after precipitation is as high as 490 mg / L, which cannot be used for the recovery of lithium in low-concentration lithium-containing solutions.

[0005] Compared with the precipitation method, the extraction method has the advantages of high lithium / sodium separation coefficient and high lithium recovery rate. The β-diketone-neutral phosphorus binary extraction system has been proven to be an effective extraction system for extracting lithium from alkaline high-sodium brine. For example, the Li / Na single-stage separation coefficient of the Lix 54 and Cyanex923 synergistic extraction system is as high as 1123. The organic phase loaded with Li is stripped with CO2 to obtain a LiHCO3 solution, which is then evaporated to obtain the LiCO3 product. However, there are two inherent defects in the β-diketone and neutral phosphorus extraction system. One is that the β-diketone and neutral phosphorus extractants are expensive, resulting in high investment costs; the other is that the β-diketone extractant has high solubility in water. Extraction and stripping need to be carried out in the aqueous phase, resulting in high operating costs and the need to increase the oil removal process. Due to the large amount of water in the low-concentration lithium-containing solution, this extraction method is not suitable for treating wastewater with low lithium content from the perspective of sustainability and cost.

[0006] Ion sieve adsorption technology is a technology that selectively recovers lithium from multi-metal mixed brine through spatial screening and memory effect, and has good selectivity. Early research focused on the selective adsorption of lithium from high magnesium Mg / Li brine. In recent years, researchers have applied it to lithium precipitation mother liquor. For example, someone has developed a new co-doped H2TiO3 spinning composite material, which can adsorb up to 40.12 mg / g of lithium in Li2CO3 precipitation mother liquor, while sodium is only 1.66 mg / g. There is also a titanium-based adsorbent that recovers lithium from Li2CO3 precipitation mother liquor through a double-column series desorption process, and the adsorption rate of lithium can reach 98%, when the lithium concentration in the desorption solution is 1.2 g / L and the sodium concentration is 24 mg / L. The ion sieve column separation process can achieve efficient Li / Na separation, but the Li concentration in the recovered solution is low, and the subsequent preparation of lithium carbonate still requires evaporation treatment, which increases the cost of lithium recovery.

[0007] Electrochemical adsorption is a separation technology for extracting lithium with a high magnesium-to-lithium ratio from salt lakes. Compared with conventional adsorption methods, electrochemical adsorption uses electrical energy to drive the selective adsorption and desorption of lithium ions in iron phosphate. This method does not require the use of toxic chemicals and is in line with the concepts of clean production and green manufacturing. However, the electrochemical adsorption method for extracting lithium still has the problem of low lithium content in the desorption solution, which limits its application in lithium recovery in resin washing wastewater. Summary of the invention

[0008] The present invention aims to provide a method for selectively recovering lithium from a low-concentration lithium-containing solution, which is based on an organic carboxylic acid extraction-precipitation method to extract lithium from low-concentration resin regeneration wastewater, thereby solving the problems existing in the process of recovering low-concentration lithium from resin regeneration wastewater, and has the advantages of simple technical route, green and clean, and low cost.

[0009] In order to achieve the above object, the present invention provides the following technical solutions:

[0010] A method for selectively recovering lithium from a low-concentration lithium-containing solution comprises the following steps:

[0011] 1) adjusting the pH value of the lithium-containing wastewater to alkaline, and then filtering it, so that the lithium concentration in the lithium-containing wastewater after the pH is adjusted is 0.2-0.8 g / L;

[0012] 2) adding an organic carboxylic acid extracting-precipitating agent to the filtrate of step 1), heating to a certain temperature and stirring to react;

[0013] 3) Cooling to a certain temperature and filtering to obtain a filtrate and a filter cake;

[0014] 4) washing the filter cake obtained in step 3), and then adding an inorganic acid for stripping the lithium loaded therein to obtain a regenerated extraction-precipitation agent and a concentrated lithium-containing solution;

[0015] 5) The concentrated lithium-containing solution in step 4) is adjusted to be alkaline, deeply impurities are removed, filtered, and the obtained filtrate is subjected to reverse precipitation to prepare battery-grade lithium carbonate.

[0016] In this application, "low concentration lithium-containing solution" refers to lithium-containing wastewater with a lithium content of 0.2 to 0.8 g / L.

[0017] According to an embodiment of the present invention, the lithium-containing wastewater is resin regeneration lithium-containing wastewater.

[0018] According to an embodiment of the present invention, the lithium concentration in the lithium-containing wastewater after adjusting the pH in step 1) is 0.2-0.4 g / L, and the Na and K concentrations are 1-10 g / L; preferably, the Na and K concentrations are 1-5 g / L.

[0019] According to an embodiment of the present invention, certain manganese, calcium, boron and magnesium ions also exist in the lithium-containing wastewater.

[0020] According to an embodiment of the present invention, in step 1), liquid alkali is used to adjust the pH value of the wastewater to 12 to 13;

[0021] According to an embodiment of the present invention, the organic carboxylic acid extraction-precipitation agent in step 2) is selected from one or more of lauric acid, myristic acid, palmitic acid and stearic acid.

[0022] According to an embodiment of the present invention, the amount of the organic carboxylic acid extraction-precipitation agent added is 0.9 to 1.2 times the molar amount of Li in the wastewater.

[0023] According to an embodiment of the present invention, the reaction temperature in step 2) is 40-90° C., the reaction time is 10-60 min, and the stirring speed is 300-600 rpm.

[0024] According to an embodiment of the present invention, the temperature during filtration in step 3) is 10-70° C., and the shorter the alkyl chain, the lower the suitable filtration temperature.

[0025] According to an embodiment of the present invention, the washing method in step 4) is rinsing or stirring, the amount of washing water is 1wt% to 10wt% of the amount of water before precipitation, and the number of washing times is 1 to 5 times.

[0026] According to an embodiment of the present invention, the inorganic acid used for stripping in step 4) includes hydrochloric acid or sulfuric acid;

[0027] According to an embodiment of the present invention, the liquid-to-solid ratio used in the stripping in step 4) is 10:8-8:10, the stripping temperature is 80-100° C., the hydrogen ion concentration in the stripping solution is 3-4 mol / L, and the stripping time is 30-120 min.

[0028] According to an embodiment of the present invention, the pH value of the concentrated lithium-containing solution in step 5) is adjusted to 11-12.

[0029] According to an embodiment of the present invention, in step 5), phosphoric acid precipitation method or resin adsorption method is used for deep impurity removal, and the precipitant used includes one or more of phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0030] According to an embodiment of the present invention, in step 5), a phosphoric acid precipitation method is used for deep impurity removal, the amount of phosphate used is 0.67 to 1 times the molar amount of calcium, the reaction temperature is controlled at 90 to 100° C., and the reaction time is 10 to 60 min; after the reaction is balanced, the pH value is controlled at 11 to 11.5.

[0031] According to an embodiment of the present invention, in step 5), a resin adsorption method is used for deep impurity removal, and the resins used include D402 (iminodiacetic acid) and D402-II (aminophosphoric acid), and the types of resins include H type, Na type and Li type.

[0032] According to an embodiment of the present invention, the adsorption method in the deep impurity removal by resin adsorption is fixed-length column adsorption, the resin dosage is 5g / L~100g / L, the aqueous phase calcium concentration is 5~200mg / L, the feed rate is 1BV~20BV / h, and the adsorption temperature is 20~70°C.

[0033] According to an embodiment of the present invention, battery-grade lithium carbonate is prepared by reverse precipitation, and its process parameters can refer to the current lithium precipitation parameters.

[0034] According to an embodiment of the present invention, the process further comprises subjecting the filtrate obtained in step 3) to reverse osmosis treatment to obtain fresh water, wherein the process parameters of the reverse osmosis treatment refer to the process parameters currently used in the industry.

[0035] Beneficial effects of the present invention:

[0036] The present invention proposes a technical process for extracting lithium from low-concentration resin regeneration wastewater based on organic carboxylic acid extraction-precipitation method, which can not only directly extract lithium from low-concentration lithium-containing solution, but also detect that the main content of lithium carbonate is as high as 99.6%. At the same time, lithium concentration and extraction-precipitation agent circulation can be achieved through organic acid stripping, avoiding the use of MVR evaporation technology for concentration, and the lithium concentrate can be precipitated to obtain battery-grade lithium carbonate products after impurities are removed. In addition, the COD of wastewater treated with extraction-precipitation agent can be lower than 150mg / L, which meets the secondary emission standard of the second category of pollutants in GB 8978-1996 "Sewage Discharge Standard". DETAILED DESCRIPTION

[0037] The present invention will be described in further detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only exemplary descriptions and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0038] Example 1

[0039] 1) The pH value of the resin regeneration wastewater was adjusted to 12.625 using 32% alkali solution. Table 1 shows the changes in metal ion concentrations before and after the adjustment.

[0040] Table 1. Changes in composition and content of resin regeneration wastewater before and after pH adjustment

[0041]

[0042] It can be seen that the Li concentration of the regenerated wastewater after pH adjustment is 295±8 mg / L, and the Na concentration is 2226±38 mg / L, and the Mg and Mn ions in the wastewater are removed at the same time.

[0043] 2) Take 10L of the filtrate, add 119.3g of stearic acid, heat to 90°C, react for 10min, and stir at 600rpm.

[0044] 3) Then the temperature is lowered to 60°C and filtered to obtain a filtrate and a filter cake.

[0045] 4) The filter cake obtained in step 3) was rinsed with 1 L of water for 5 times.

[0046] Then sulfuric acid was added to strip the lithium loaded therein, the liquid-solid ratio used for stripping was 10:8, the stripping temperature was 99°C, the hydrogen ion concentration in the stripping solution was 3.8 mol / L, and the stripping time was 60 min; the regenerated extraction-precipitation agent and the concentrated lithium-containing stripping solution were obtained. At this time, the lithium concentration in the lithium-containing stripping solution was 19.18 g / L, the sodium concentration was 4.42 g / L, and the calcium concentration was 139 mg / L.

[0047] 5) Add 0.033 g of disodium hydrogen phosphate to the lithium-containing stripping solution, stir to dissolve it, and then adjust the pH to 11 with liquid alkali, boil and react for 10 minutes to form calcium phosphate precipitation. After cooling to room temperature, filter the precipitate in the lithium-containing stripping solution, and the calcium content therein can be reduced to 11 mg / L.

[0048] 6) The stripping solution after decalcification was slowly added into an exothermic sodium carbonate solution (280 g / L) at a temperature of 100° C. using a constant pressure funnel to obtain a crude lithium carbonate product. The crude lithium carbonate product was stirred and washed 3 times under a liquid-to-solid ratio of 5 to obtain a finished lithium carbonate product. The main lithium carbonate content was 99.6% when detected by ICP.

[0049] Embodiment 2-4

[0050] The preparation method is the same as that of Example 1, except that the feed solution (Li, Na, K concentrations are different) is different, which has a certain influence on the extraction-precipitation rate.

[0051] 0.288 mmol of tetradecanoic acid, palmitic acid or stearic acid was added to 10 mL of lithium-containing wastewater with a pH of 12.46, a Li concentration of 0.2 g / L, and a Na / K concentration of 1 g / L, 5 g / L or 10 g / L; the reaction was carried out at 75 °C for 30 min, and the mixture was cooled to room temperature and filtered. Table 2 shows the precipitation rate of carboxylic acids for lithium ions and their residues in water under different parameters.

[0052] Table 2. Effect of Na / K concentration on lithium ion precipitation by carboxylic acid

[0053]

[0054] It can be seen from Table 2 that myristic acid, palmitic acid and stearic acid are all superior to sodium and potassium in the precipitation order of lithium, and can selectively precipitate more than 55% of lithium from a high sodium and potassium solution.

[0055] When the feed liquid composition is Li: 0.2g / L, Na: 10g / L, K: 10g / L, when myristic acid, palmitic acid and stearic acid are used to precipitate Li, the Li precipitation rates are 60%, 58% and 55% respectively, which are all low, indicating that organic carboxylic acids are not suitable for treating lithium-containing wastewater with excessively high Na and K contents. The concentrations of Na and K need to be controlled within a certain range to better precipitate lithium.

[0056] When the sodium potassium concentration is 1g / L and 5g / L, the selectivity of the three carboxylic acids on lithium is tetradecanoic acid < palmitic acid < stearic acid. When the sodium potassium concentration is 10g / L, the selectivity of the three carboxylic acids on lithium is tetradecanoic acid > palmitic acid > stearic acid. From the COD value of the liquid after precipitation, the residual carbon chain of the carboxylic acid in the water decreases with the increase of the metal ion concentration in the water phase, and the COD value of the liquid after lithium precipitation can be lower than 150mg / L.

[0057] Embodiment 5-8

[0058] The preparation method is the same as that of Example 1, except for the filtration temperature, the amount of precipitant and the pH value of the feed solution.

[0059] Stearic acid was added to 10 mL of resin regeneration wastewater (reference example 1 for feed liquid composition), reacted at 75°C for 10 min, and filtered after being kept warm to a certain temperature. The effects of filtration temperature, precipitant dosage, and feed liquid pH value on the precipitation rates of Li, Na, and Ca ions are shown in Table 3.

[0060] Table 3

[0061] It can be seen from Table 3 that with the increase of filtration temperature, the precipitation rate of stearic acid for Li increases, and the precipitation rate for Na decreases, indicating that hot filtration is beneficial to improving the separation effect of Li and Na. The amount of precipitant and the pH of the feed solution have little effect on the precipitation rate of Li and the separation effect of Li / Na.

[0062] Example 9

[0063] 11.93 g of stearic acid was added to 1 L of resin regeneration wastewater (the feed liquid composition was the same as in Example 1) adjusted to alkalinity, heated to 85° C. for reaction for 30 min, then cooled to 60° C. and filtered while keeping warm.

[0064] The filter cake was rinsed with 250 mL of cold water for 5 times. After washing, 15 mL of 1.8 mol / L dilute sulfuric acid was added and stripped at 95 °C for 30 min to strip off the lithium loaded therein. After cooling, the solidified precipitant was taken out and 15 mL of 0.05 mol / L dilute sulfuric acid was added for secondary stripping. Finally, pure water was added and stirred for 30 min to obtain regenerated stearic acid. After replenishing the loss during use, it was repeatedly added to the resin regeneration wastewater to recover lithium.

[0065] Table 4 shows the precipitation rates of Li, Na and Ca ions with the number of SA cycles in different cycles, and Table 5 shows the stripping rates of metal ions in different cycles.

[0066] Table 4

[0067]

[0068]

[0069] Table 5. Effect of cycle number and stripping number on Li, Na and Ca ion stripping

[0070]

[0071] It can be seen from Table 4 that stearic acid can recover its precipitation performance after secondary stripping with sulfuric acid and finally washing with water three times. During the five cycles of use, its precipitation performance for lithium and separation performance of Li / Na and Li / Ca did not change significantly, indicating that stearic acid can be recycled.

[0072] As can be seen from Table 5, the concentrations of Li, Na and Ca in the three stripping solutions are all lower than 1 mg / L, indicating that after the second stripping with sulfuric acid, more than 99.9% of Li, Na and Ca in the precipitate enter the stripping solution. The proportions of Li, Na and Ca in the first stripping solution are 91.2-99.4%, 91.4%-99.5% and Ca92.4-98.7% respectively.

[0073] Example 10

[0074] The preparation method is the same as that of Example 9, except that:

[0075] Take the primary stripping solution prepared in Example 9, adjust the pH to 11, and take 600 mL at a feed flow rate of 1 BV / h, and flow it from top to bottom through an adsorption column (bed volume of 30 mL, column inner diameter of 11 mm) filled with Sphelite ACD-500 resin. The calcium content of the effluent is less than 3 mg / L. Subsequently, 150 mL of the effluent is added to a Na2CO3 solution with a temperature of 100°C and a concentration of 280 g / L at a feed flow rate of 0.15 L / h. To obtain a slurry containing lithium carbonate.

[0076] The slurry was stirred and aged for 30 minutes, and the lithium carbonate product was obtained after filtering, washing and drying. The equilibrium pH was adjusted to 11.3, boiled for 10 minutes, cooled to room temperature and filtered. The calcium content in the stripping solution can be reduced to 3-10 mg / L. Then, lithium was reversed and precipitated according to the industrial method to obtain lithium carbonate, which was stirred and washed 3 times under the condition of a liquid-to-solid ratio of 5 to obtain the finished lithium carbonate product. The main content of lithium carbonate was detected by ICP and was 99.6%.

[0077] The above is an explanation of the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for selectively recovering lithium from a low-concentration lithium-containing solution, characterized in that: The following steps are involved: 1) adjusting the pH value of the lithium-containing wastewater to alkaline, and then filtering it, so that the lithium concentration in the lithium-containing wastewater after the pH is adjusted is 0.2-0.8 g / L; 2) adding an organic carboxylic acid extracting-precipitating agent to the filtrate of step 1), heating to a certain temperature and stirring to react; 3) Cooling to a certain temperature and filtering to obtain a filtrate and a filter cake; 4) washing the filter cake obtained in step 3), and then adding an inorganic acid for stripping the lithium loaded therein to obtain a regenerated extraction-precipitation agent and a concentrated lithium-containing solution; 5) The concentrated lithium-containing solution in step 4) is adjusted to be alkaline, deeply impurities are removed, filtered, and the obtained filtrate is subjected to reverse precipitation to prepare battery-grade lithium carbonate.

2. The method according to claim 1, characterized in that The lithium-containing wastewater is resin regeneration lithium-containing wastewater; Preferably, the lithium concentration in the lithium-containing wastewater after adjusting the pH in step 1) is 0.2-0.4 g / L, and the Na and K concentrations are 1-10 g / L; more preferably, the Na and K concentrations are 1-5 g / L; Preferably, the lithium-containing wastewater also contains certain manganese, calcium, boron and magnesium ions; For example, the concentrations of lithium, sodium and potassium in lithium-containing wastewater after pH adjustment are: Li: 0.2g / L, Na: 1g / L, K: 1g / L; Li: 0.2g / L, Na: 5g / L, K: 5g / L; Li: 0.2g / L, Na: 10g / L, K: 10g / L.

3. The method according to claim 1, characterized in that In the step 1), liquid alkali is used to adjust the pH value of the wastewater to 12-13; Preferably, the organic carboxylic acid extraction-precipitation agent in step 2) is selected from one or more of lauric acid, myristic acid, palmitic acid and stearic acid; The amount of the organic carboxylic acid extraction-precipitation agent added is 0.9 to 1.2 times the molar amount of Li in the wastewater.

4. The method according to claim 1, characterized in that: The reaction temperature in step 2) is 40-90° C., the reaction time is 10-60 min, and the stirring speed is 300-600 rpm.

5. The method according to claim 1, characterized in that: The filtration temperature in step 3) is 10-70° C., and the shorter the alkyl chain, the lower the suitable filtration temperature.

6. The method according to claim 1, characterized in that The washing method in step 4) is rinsing or stirring, the amount of washing water is 1wt% to 10wt% of the water before precipitation, and the number of washing times is 1 to 5 times; Preferably, the inorganic acid used for stripping in step 4) includes hydrochloric acid or sulfuric acid; Preferably, the liquid-to-solid ratio used in the stripping in step 4) is 10:8-8:10, the stripping temperature is 80-100° C., the hydrogen ion concentration in the stripping solution is 3-4 mol / L, and the stripping time is 30-120 min.

7. The method according to claim 1, characterized in that The pH value of the concentrated lithium-containing solution in step 5) is adjusted to 11 to 12; Preferably, in step 5), phosphoric acid precipitation method or resin adsorption method is used for deep impurity removal, and the precipitant used includes one or more of phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

8. The method according to claim 1, characterized in that: Preferably, in step 5), a phosphoric acid precipitation method is used for deep impurity removal, the amount of phosphate used is 0.67 to 1 times the molar amount of calcium, the reaction temperature is controlled at 90 to 100° C., the reaction time is 10 to 60 min; and the pH value is controlled at 11 to 11.5 after the reaction is balanced.

9. The method according to claim 1, characterized in that: In the step 5), a resin adsorption method is used for deep impurity removal, and the resins used include iminodiacetic acid and aminophosphoric acid, and the types of the resins include H type, Na type and Li type; Preferably, the adsorption method in the resin adsorption method for deep impurity removal is fixed-length column adsorption, the resin dosage is 5g / L~100g / L, the aqueous phase calcium concentration is 5~200mg / L, the feed rate is 1BV~20BV / h, and the adsorption temperature is 20~70°C.

10. The method according to claim 1, characterized in that The method also includes subjecting the filtrate obtained in step 3) to reverse osmosis treatment to obtain fresh water.

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