CN108341420B

CN108341420B - Method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with high magnesium-lithium ratio

Info

Publication number: CN108341420B
Application number: CN201710059740.6A
Authority: CN
Inventors: 马培华
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-01-24
Filing date: 2017-01-24
Publication date: 2022-02-08
Anticipated expiration: 2037-01-24
Also published as: CN108341420A; AR108604A1

Abstract

The invention discloses a method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with high magnesium-lithium ratio, which comprises the following process steps: (1) after the potassium is extracted from the salt pan, the brine is further stabilized in a stabilization tank to form low-potassium-sodium boron-lithium brine; (2) carrying out boron extraction treatment on the boron-lithium brine to form a boric acid product and lithium brine; (3) refining lithium brine for three times to obtain refined liquid for three times; (6) the third refined solution is processed by a bipolar membrane electrodialyzer to form a lithium hydroxide solution; (7) passing the lithium hydroxide solution through an evaporative crystallizer to obtain a lithium hydroxide monohydrate solid and an evaporative mother liquor; (8) recrystallizing the lithium hydroxide monohydrate solid by washing to form battery-grade lithium hydroxide and a washing solution; (9) and reacting the evaporation mother liquor and the washing liquid with carbon dioxide gas through a gas-liquid reactor to form lithium carbonate. The method has good operability, and greatly improves the recovery rate of the lithium ions.

Description

Method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with high magnesium-lithium ratio

Technical Field

The invention belongs to the field of inorganic salt chemical industry, and particularly relates to a method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with a high magnesium-lithium ratio.

Background

Lithium is the most important energy metal and an indispensable strategic resource in modern industry, and plays an important role in battery chemistry, glass ceramics, aviation metal, nuclear industry, lubricating grease, refrigerant and the like. Especially, the explosive development of lithium batteries in recent years has led to a rapid expansion of the global lithium consumption. The global lithium production capacity is 80% from salt lakes, the easily developed salt lakes with low magnesium-lithium ratio are fully exploited, and the rapidly growing lithium demand urgently needs to extract battery-grade lithium carbonate from the salt lakes with high magnesium-lithium ratio so as to meet the demand of the battery industry.

Most of the global high magnesium-lithium ratio salt lake brine has the characteristics of high sulfate radical and boron and high sodium-potassium content. The salt lake brine is concentrated by solarization in a salt pan, and has three obvious stages: salt crystallization, carnallite crystallization and picromerite crystallization, wherein the carnallite crystallization and the picromerite crystallization are respectively used for producing potassium chloride and potash magnesium sulphate fertilizers. After the picromerite is crystallized, lithium and boron in the salt lake brine are highly enriched.

According to different resource endowments, the concentration of lithium ions is between 0.3g/L and 6g/L, the total concentration of potassium and sodium ions is over 7g/L, the concentration of sulfate radicals is over 25g/L, and the concentration of magnesium ions is generally higher than 110g/L and is close to magnesium chloride saturation.

Further solarization and concentration of the brine can separate out a large amount of bischofite to cause huge entrainment loss of lithium. Meanwhile, during the concentration process, high sulfate radicals can cause the sedimentation loss of lithium sulfate in the salt pan. In conclusion, it can be seen that the lithium ion concentration of the salt lake brine with high magnesium-lithium ratio determines the magnesium-lithium ratio of the brine after concentration in the salt pan, and further evaporation concentration of the salt pan concentrated brine after magnesium saturation can cause a great amount of lithium ion loss.

In the existing process for extracting lithium from salt lake, because of various defects of magnesium-lithium separation process, in order to reduce production cost of lithium extraction workshop, a method for further naturally evaporating and concentrating to improve lithium ion concentration in brine is adopted, so that the lithium ion yield in the process of salt field is reduced to below 10%. Namely, the deficiency of the magnesium-lithium separation process in the workshop leads to the waste of a large amount of lithium resources in the salt lake.

Therefore, in the global development of lithium resources in salt lakes with high magnesium-lithium ratio, a technology for extracting lithium carbonate from brine with high magnesium-lithium ratio, which can deal with the magnesium-lithium separation technology under complex brine conditions, is urgently needed.

In the process of extracting lithium from a salt lake with a high magnesium-lithium ratio, the following process methods are actually applied at present:

(1) adsorption process

Adsorbing lithium ions by using an adsorbent selective for lithium ions, eluting the lithium ions by using fresh water or dilute acid solution, wherein the magnesium-lithium ratio in the comprehensive eluent is lower than 3: 1, the primary magnesium-lithium separation effect can be achieved.

However, the adsorption method consumes a large amount of valuable fresh water in the salt lake region, the concentration of lithium ions in eluent is low and is usually lower than 0.5g/L, and the multi-stage concentration of the back-end process causes complex operation and high-cost operation. Meanwhile, the adsorbent has the problems of high price, high possibility of being polluted in high-magnesium brine to cause rapid reduction of adsorption capacity and the like, and the adsorbent is easy to damage and corrode and cannot be recycled for a long time.

(2) Calcining process

Evaporating and drying the lithium-enriched brine to obtain dry powder with complex components, and calcining the dry powder at high temperature to convert alkaline magnesium chloride in the dry powder into magnesium oxide and hydrogen chloride gas. And leaching the calcined solid with fresh water to obtain a lithium chloride solution. Because magnesium oxide is insoluble in water, the calcining method has good magnesium-lithium separation effect.

However, the calcining method consumes a large amount of natural gas, hydrogen chloride gas generated in the calcining process has strong corrosivity to equipment and pollutes the environment, and lithium in waste residues is seriously entrained. The cost of the calcining process is high due to the tail gas treatment process and the huge energy consumption.

(3) Electrodialysis method

The lithium-containing brine passes through a multi-stage ion selective electrodialyzer, magnesium sulfate is intercepted under the action of electric field force by utilizing the good intercepting effect of an ion selective membrane on divalent anions and cations, and lithium chloride passes through the ion selective membrane to form a lithium-rich solution.

The electrodialysis method has no pollution and controllable cost, and is the only method for realizing continuous large-scale production in the existing lithium extraction process of the salt lake with high magnesium ratio. However, the water consumption and the power consumption of the multi-stage electrodialyzer are high, the concentration of magnesium ions in the formed lithium-rich brine is high and reaches more than 10g/L, a large amount of caustic soda solution is used for removing magnesium and then hydrochloric acid is used for leveling the pH value, and a large amount of sodium ions are introduced to cause high-cost operation of the subsequent evaporation concentration and lithium precipitation processes.

In addition to the three actually used processes for extracting lithium from salt lakes with high magnesium ratio, the method disclosed in the Chinese patent applications with application numbers of 03108088.X and 201310571755.2, etc. all of which use nanofiltration membranes to separate magnesium and lithium. However, in these patent applications, magnesium-saturated lithium brine (magnesium ion concentration is more than 115 g/L) is used in an overlapping manner to perform nanofiltration treatment after ten times of fresh water dilution, and these process methods all cause a large amount of fresh water consumption and increase the volume of the lithium brine by tens of times. Under the condition that the nanofiltration membrane magnesium-lithium separation has no good concentration effect, the concentration of lithium ions in the nanofiltration membrane produced water is lower than 0.5g/L, even lower than the concentration of eluent in the adsorption process, and the overall operation cost is higher than that of the adsorption process.

The use of calcium chloride to reduce the concentration of sulfuric acid in boron lithium brine to avoid the precipitation loss of lithium sulfate is mentioned in chinese patent application 201310571755.2, but this method cannot solve the entrainment loss caused by the precipitation of a large amount of bischofite in the salt pan, and the several-fold concentration of magnesium saturated brine necessarily causes the complete loss of liquid lithium brine in the concentration tank due to the precipitation and entrainment of bischofite. Meanwhile, calcium pollution is introduced, which is extremely unfavorable for subsequent process operation, and causes the complexity of salt field operation and the cost increase of processes such as calcium chloride purchase, transportation and liquid preparation, addition, mixing and the like.

The difference of battery-grade lithium carbonate required by the lithium battery anode material industry in China is large, and industry or local battery-grade standards are established by association and areas of various industries respectively. The manufacturing requirements and manufacturing processes of different battery materials are different, so that the Chinese battery grade lithium carbonate has no universal standard all the time. The battery-grade lithium carbonate adopted in the current China battery positive electrode material industry is formed by secondary processing and purification of industrial-grade lithium carbonate. After the actual control standards of each battery material factory are integrated, the battery level control standard used for the method is provided.

The battery grade lithium carbonate standard generally required by the cathode material of the Chinese lithium battery is as follows:

disclosure of Invention

The invention provides a lithium carbonate production process for producing lithium carbonate meeting the general requirements of the existing lithium battery materials in China in salt lake brine with a high magnesium-lithium ratio without secondary purification.

According to one aspect of the invention, the invention relates to a method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with high magnesium-lithium ratio, which comprises the following process steps:

(1) the method comprises the following steps of (1) naturally evaporating brine obtained after potassium is extracted from a salt pan in a stabilizing tank by utilizing solar energy to reduce the total concentration of sodium and potassium ions in the brine, generating a potassium fertilizer raw material in an adjusting tank, and naturally evaporating and concentrating the potassium fertilizer raw material to form concentrated boron-lithium brine with stable components (the control key points are that firstly, the sedimentation loss of lithium sulfate in the adjusting tank caused by overhigh concentration of sulfate radicals is prevented, and secondly, the entrainment loss of the boron-lithium-rich brine caused by the deposition of a large amount of bischofite is prevented);

(2) roughly filtering the boron-lithium brine, mixing the boron-lithium brine and industrial hydrochloric acid in a high-efficiency reactor in a fixed proportion for acidification reaction, cooling and filtering to form crude boric acid solid and acidified liquid, enabling the acidified liquid and a no-load extractant to enter a high-efficiency centrifugal extractor, extracting and deborizing the acidified liquid to form lithium brine, enabling a loaded extractant and fresh water to form a boric acid solution through a high-efficiency centrifugal back-extraction process, and heating, dissolving and cooling and crystallizing the boric acid solution and the crude boric acid to obtain refined boric acid;

(3) the lithium brine is made to pass through an electrodialyzer with a monovalent ion selection function, divalent ions in the lithium brine are intercepted by an ion selection membrane, monovalent ions in the lithium brine permeate the ion selection membrane, magnesium and lithium separation is achieved, and a primary refined liquid is obtained, wherein the primary refined liquid is a lithium ion concentrate, and the magnesium-lithium ratio is lower than 2: 1; returning the tail liquid with high magnesium and low lithium to a salt pan;

(4) directly filtering the primary refined liquid by a nanofiltration membrane, wherein more than 90% of magnesium ions and sulfate radicals are intercepted to obtain secondary refined liquid and concentrated water, the concentrated water returns to the lithium brine to complete the dilution of the lithium brine, meanwhile, the lithium ions in the concentrated water are recovered by diluting the lithium brine in the step (3), and the concentration of the magnesium ions in the secondary refined liquid is lower than 1 g/L;

(5) removing calcium, magnesium and sulfate radicals from the secondary refined liquid by a calcium and magnesium ion selective cation resin and a divalent anion selective resin ion exchanger to obtain a tertiary refined liquid, wherein the concentration of the calcium, magnesium ions and sulfate radicals is lower than 20 ppm;

(6) feeding the three refined solutions into desalting chamber of bipolar membrane electrodialyzer, and under the action of electric field force, Li in the three refined solutions⁺Passes through the anode membrane to the base chamber and reacts with OH generated by the electrolysis of water through the bipolar membrane^-Pairing to form lithium hydroxide solution, and refining for three timesX in liquid^-Passes through the vaginal membrane to the acid compartment and reacts with H generated by the electrolysis of water through the bipolar membrane⁺Pairing to form HX solution;

the bipolar membrane electrodialyzer is in a five-chamber form, the five chambers are an anode chamber, an alkali chamber, an acid chamber, a desalting chamber and a cathode chamber, wherein a special cation exchange membrane, the alkali chamber, the bipolar membrane, the acid chamber, an anion exchange membrane, the desalting chamber and the cation exchange membrane are sequentially overlapped in multiple groups to form a three-chamber membrane group, an anode plate is arranged in the anode chamber, a cathode plate is arranged in the cathode chamber, and the membrane group is clamped between the anode chamber and the cathode chamber, so that the bipolar membrane electrodialyzer is formed;

during the continuous operation of the bipolar membrane electrodialyzer, four solutions are respectively circulated in the bipolar membrane electrodialyzer, wherein the four solutions are respectively: feed liquid, acid liquid, alkali liquid and polar liquid, wherein the feed liquid is the third refined liquid,

the stable circulating operation of the feed liquid in the desalting chamber of the bipolar membrane electrodialyzer is realized through a feed liquid balance buffer tank, and meanwhile, the continuous supplement of the feed liquid amount and the balanced discharge of diluent are realized;

the stable circulating operation of the acid liquid in an acid chamber of the bipolar membrane electrodialyzer is realized through an acid liquid balance buffer tank, and meanwhile, the continuous supplement of the acid liquid amount and the balanced discharge of concentrated acid liquid are realized;

the stable circulating operation of the alkali liquor in an alkali chamber of the bipolar membrane electrodialyzer is realized through an alkali liquor balance buffer tank, and meanwhile, the continuous supplement of the alkali liquor amount and the balanced discharge of concentrated alkali liquor are realized;

directly using concentrated alkali liquor in an alkali liquor buffer tank in an alkali liquor balance buffer tank as polar liquor to realize stable circulating operation of the polar liquor in a polar chamber of the bipolar membrane electrodialyzer, and overflowing from the alkali liquor buffer tank to obtain a lithium hydroxide solution;

(7) forcibly evaporating and concentrating the lithium hydroxide solution obtained by overflowing through an evaporation crystallizer, continuously discharging a mixture of a lithium hydroxide monohydrate solid and an evaporation mother liquor from a salt discharge angle of the evaporation crystallizer, and filtering the mixture to respectively obtain the lithium hydroxide monohydrate solid and the evaporation mother liquor, wherein the evaporation mother liquor is the lithium hydroxide solution;

(8) carrying out washing recrystallization on the lithium hydroxide monohydrate solid to form battery-grade lithium hydroxide and a washing solution, wherein the washing solution is a lithium hydroxide solution;

(9) and mixing the evaporation mother liquor with the washing liquor, allowing the mixture to pass through a gas-liquid reactor, reacting with carbon dioxide gas in the reactor to form lithium carbonate sediment, and filtering, washing and drying the lithium carbonate sediment to obtain the battery-grade lithium carbonate.

Further, in the step (1), the brine after potassium extraction is further concentrated in a stabilization tank to form low-potassium-sodium boron-lithium brine, wherein the total potassium-sodium ion content is lower than 5g/L, and the product of the lithium ion concentration and the sulfate radical concentration is less than 150g²/L²。

Further, in the step (2), the boron-lithium brine and hydrochloric acid are subjected to acidification reaction in a high-efficiency reactor in a certain proportion to generate boric acid, and crude boric acid is obtained by filtering, wherein the pH value of the mixed solution is 1-4.

Further, in the step (2), the acidified solution and the no-load extractant pass through a centrifugal extractor, boric acid in the acidified solution is extracted to form lithium brine, the boric acid content in the lithium brine is lower than 0.6g/L, a boric acid solution is formed after the loaded extractant and fresh water pass through the centrifugal extractor, and the loaded extractant is subjected to boron removal through the centrifugal extractor to form the no-load extractant and is reused.

Further, in the step (2), the boric acid solution formed by the back extraction and the crude boric acid formed by the acidification reaction of the boron-lithium brine are heated and dissolved to form a saturated boric acid solution, the temperature of the saturated boric acid solution is 30-80 ℃, and the refined boric acid is obtained by a cooling crystallization process.

Further, in the step (3), the lithium brine passes through an electrodialyzer equipped with a monovalent ion selective membrane, monovalent ions in the lithium-rich brine permeate the ion selective membrane under the action of an electric field force, and divalent ions in the lithium brine are intercepted by the ion selective membrane, so that magnesium and lithium separation and lithium concentration are simultaneously completed, and a primary refined liquid with high lithium content and low magnesium content is formed. Representative monovalent ion selective membranes include K-192 and ACS-8T membranes from ASTOM and other monovalent ion selective membranes having similar properties.

Further, in the step (4), a nanofiltration membrane with a barrier effect on calcium and magnesium ions is selected, wherein representative nanofiltration membranes comprise DK membranes and nanofiltration membranes with similar performance produced by other companies, the primary refined liquid enters a nanofiltration device, the magnesium-lithium ratio is further reduced, and a secondary refined liquid is obtained, wherein the concentration of magnesium ions on the concentrated water side of the nanofiltration membrane is lower than 35g/L for the primary refined liquid entering the nanofiltration membrane process device.

Further, in the step (5), the bipolar membrane electrodialyzer is a membrane group in the form of three chambers of a cation exchange membrane, an alkali chamber, a bipolar membrane, an acid chamber, an anion exchange membrane, a desalting chamber and a cation exchange membrane in combination between an anode chamber and a cathode chamber,

between the anode side and the cathode side are an anode chamber, N × (alkali chamber + acid chamber + desalination chamber), a cathode chamber, where N is adjusted between 30-300 according to the design yield,

wherein a corrosion-resistant special cation exchange membrane is arranged between the anode chamber and the alkali chamber so as to avoid the membrane damage caused by the oxidation corrosion of the cation exchange membrane by oxygen atoms generated during the anode reaction.

Furthermore, an anode plate is arranged in the anode chamber of the bipolar membrane electrodialyzer, a cathode plate is arranged in the cathode chamber,

the anode plate is made of titanium-coated iridium oxide, the cathode plate is made of titanium or stainless steel,

the anode plate is connected to the positive pole of a direct current power supply through a cable, and the cathode plate is connected to the negative pole of the direct current power supply through a cable.

Furthermore, the balance buffer groove is designed in a double-groove parallel connection mode, the double grooves are a balance groove and a buffer groove, the balance groove and the buffer groove are communicated through an opening at the bottom of the groove partition plate,

the top of the balancing tank is provided with a supplementary liquid inlet, an outlet pipe at the bottom of the balancing tank is connected with a circulating pump,

the top of the buffer tank is a circulating liquid inlet, and an overflow outlet is arranged at the liquid level position at the top of the buffer tank.

Further, the feed liquid is fed into a feed liquid balancing tank, wherein the feed liquid is continuously added from a top inlet of the feed liquid balancing tank, the feed liquid is discharged from a bottom outlet of the feed liquid balancing tank, the feed liquid is pressurized by a feed liquid pump and enters a desalting chamber of the bipolar membrane electrodialyzer,

the feed liquid loses part of electrolyte in a desalting chamber of the bipolar membrane electrodialyzer so as to obtain a diluent, the diluent is added from a top inlet of a feed liquid buffer groove and enters a feed liquid balancing groove from a bottom opening of the feed liquid buffer groove to form feed liquid circulation,

along with the addition of the feed liquid, the liquid level of the feed liquid balance buffer tank integrally rises, and the diluent returned by the bipolar membrane electrodialyzer overflows to the acid liquid balance tank from an upper overflow hole of the feed liquid buffer tank.

Further, the diluent overflowing from the feed liquid buffer tank is continuously added from a top inlet of the acid liquor balance tank,

the dilute acid liquid in the acid liquid balancing tank enters an acid chamber of the bipolar membrane electrodialyzer from the bottom outlet of the acid liquid balancing tank through the pressurization of an acid liquid pump,

forming concentrated acid liquor in an acid chamber of the bipolar membrane electrodialyzer, wherein the concentrated acid liquor is added from a top inlet of an acid liquor buffer tank and enters an acid liquor balancing tank from a bottom opening of the acid liquor buffer tank to form acid liquor circulation,

and (3) along with the addition of the diluent, the liquid level of the acid liquor balance buffer tank integrally rises, and the concentrated acid liquor overflows from a liquid level overflow hole of the acid liquor buffer tank and is then recycled.

Further, deionized water is continuously added from the top inlet of the alkali liquor balancing tank,

discharging dilute alkali liquor from a bottom outlet of the alkali liquor balancing tank, pressurizing the dilute alkali liquor by an alkali liquor pump, and feeding the dilute alkali liquor into an alkali chamber of the bipolar membrane electrodialyzer,

forming concentrated alkali liquor in an alkali chamber of the bipolar membrane electrodialyzer, wherein the concentrated alkali liquor is added from a top inlet of an alkali liquor buffer tank and enters an alkali liquor balancing tank from a bottom opening of the alkali liquor buffer tank to form alkali liquor circulation,

along with the addition of deionized water, the liquid level of the alkali liquor balance buffer tank integrally rises, and the concentrated alkali liquor overflows from a liquid level overflow hole of the alkali liquor buffer tank and then enters an evaporation process.

Further, the lithium hydroxide solution enters an electrode chamber of the bipolar membrane electrodialyzer from a bottom outlet of the alkali liquor buffer tank through the pressurization of an electrode solution pump, and the electrode solution returns from a top inlet of the alkali liquor balance tank to form electrode solution circulation.

Further, in the step (7), the lithium hydroxide solution is evaporated and concentrated through an evaporation crystallizer, a mixture of lithium hydroxide monohydrate solid and evaporation mother liquor is discharged from a salt discharge angle of the evaporation crystallizer,

centrifugally filtering the mixture to respectively obtain a lithium hydroxide monohydrate solid and an evaporation mother liquor,

and washing and recrystallizing the lithium hydroxide monohydrate solid to form the battery-grade lithium hydroxide monohydrate and a washing solution.

Further, carrying out carbonization reaction on evaporation mother liquor generated by centrifugal filtration and washing liquor generated in the washing and recrystallization processes and carbon dioxide gas in a gas-liquid reactor, controlling the reaction temperature at 25 ℃ and the pH value at the end point of the reaction at 12 to form lithium carbonate precipitate,

washing, filtering and drying the lithium carbonate precipitate to form the battery grade lithium carbonate.

Furthermore, the base layer of the special cation exchange membrane is made of a corrosion-resistant material.

Furthermore, the corrosion-resistant material is polytetrafluoroethylene.

The process of the invention has the following steps (see fig. 3):

(1) the boron-lithium brine is formed by controlling the evaporation amount of the brine in a stabilization tank after potassium extraction

In the existing lithium extraction process, one of the following two measures is adopted: (i) for the salt lake brine containing lithium, the picromerite in the salt pan process is crystallized and then directly filtered for use. This results in a high potassium/sodium content in the raw material lithium brine entering the lithium extraction plant, which leads to a significant increase in the operating cost and energy consumption of the process solution and product pollution when the process solution is run directly in the production of the plant. Or, (ii) further concentrated by a complex solarization concentration process with substantial settling of bischofite, sacrificing substantial li-ion entrainment losses and settling losses.

The invention has good magnesium-lithium separation effect and low cost in the magnesium-lithium separation process, and has good effect and cost performance in the treatment of the brine with high magnesium-lithium ratio. The existing potash fertilizer raw material salt pan process can be directly utilized, brine after potassium extraction is led into a regulating tank at the tail part of a picromerite crystallization stage for stable concentration, and the sulfate radical concentration is controlled according to the environmental temperature before lithium sulfate is settled and before bischofite is largely settled. Greatly reduces the operation difficulty of the salt pan and greatly improves the lithium ion yield in the process of the salt pan. According to the operation experience, in the regulating tank, under the condition of not needing auxiliary agents, the total amount of potassium and sodium ions is controlled to be lower than 5g/L, and the product of the concentration of lithium ions and the concentration of sulfate radicals is controlled to be less than 150g²/L²So as to ensure the yield of the lithium ions in the salt pan under the condition of potassium-lithium coproduction.

The stabilizing pool is simultaneously used as a boron-lithium brine storage pool, salt field brine is discharged from the stabilizing pool, the evaporation concentration state of the salt field brine is adjusted according to the environmental state, and the boron-lithium brine with stable components is kept for storage.

(2) Boron removal of boron from lithium-boron brine to form lithium brine and refined boric acid

The concentration of boron in the boron-lithium brine is 2-8 g/L. Heating brine to 30-40 ℃ by using industrial steam in a temperature regulating tank, fully mixing the brine with industrial hydrochloric acid with the content of 20-25% in a high-efficiency reactor, reacting, cooling the reaction liquid by using crude boric acid to form crude boric acid crystals, filtering by using a plate filter to obtain crude boric acid solids and acidified liquid, and reducing the boron concentration of the boron-lithium brine to about 2g/L after the boron-lithium brine is subjected to acidification reaction and filtering.

And (4) passing the acidified solution and a boric acid extracting agent through a centrifugal extractor. And in a centrifugal extractor, fully mixing the acidified solution with a no-load extractant, and extracting boric acid in the acidified solution to respectively form a light-phase loaded extractant and a heavy-phase lithium brine. The heavy-phase lithium brine is deoiled to form a finished product lithium brine, wherein the boron content in the lithium brine is lower than 0.4g/L, and the pH value is about 1-4.

The loaded extractant and fresh water are fully mixed and extracted in a back extraction machine to respectively form a heavy-phase boric acid solution and a light-phase no-load extractant, and the extractant is recycled after dehydration. The content of boron in the boric acid solution is 7g/L, the boric acid solution and the crude boric acid are subjected to hot-melt cold crystallization to produce a refined boric acid product, and the boric acid mother solution is returned to be recycled before acidification and is simultaneously used for diluting the boron-lithium brine.

The technological process of acidification and extraction of boron is shown in figure 1.

(3) The lithium bittern is passed through ion selective electric dialyzer to form one-pass refined liquid

The lithium brine after boron extraction is diluted by the concentrated water returned by the back-end nanofiltration process, the concentration of magnesium ions is 50-100 g/L, the content of boron is 0.1-0.4 g/L, and the concentration of lithium ions is 0.3-6 g/L. The brine is very suitable for electrodialysis process. The ion selective membrane has good boron interception effect, the content of boron in the lithium brine is low, and the filtration and electrodialysis processes are not influenced. The electrodialysis process is different from the electrolysis process, and chlorine and hydrogen are not generated in the process.

After being filtered, the lithium brine enters a dilution chamber of monovalent ion selective membrane electrodialysis equipment. The pure water enters a concentration chamber of the electrodialysis device. Under the action of electric field force, monovalent cation (Li)⁺) From the diluting compartment through the cation membrane to the concentrating compartment, and divalent cations (Mg)²⁺) Intercepted in the dilution chamber by the ion selective membrane. Monovalent anion (Cl)^-) Passes through the negative membrane to the concentrating compartment, and the divalent anions (SO)₄ ^2-) Intercepted in the dilution chamber by the ion selective membrane. The positive membrane blocks the passage of anions, as does the negative membrane. After circulation, the concentrated solution forms a primary refined solution, wherein the concentration of lithium ions is higher than 10g/L, and the concentration of magnesium ions is lower than 10 g/L. The concentration of lithium ions in the diluent is lower than 0.3g/L, and the concentration of magnesium ions in the diluent is higher than 50 g/L. Discharging the diluent to a salt pan for brine mixing operation, and recycling the boron-lithium brine formed in the concentration process of the lithium ions in the discharged liquid in the salt pan.

The principle of ion-selective electrodialysis for magnesium-lithium separation is shown in fig. 2.

(4) The primary refined liquid is subjected to magnesium removal by a nanofiltration device to form secondary refined liquid

In the primary refined liquid after the ion selective electrodialysis treatment, the concentration of lithium ions is higher than 10g/L, and the concentration of magnesium ions is lower than 10 g/L. Because the magnesium ion concentration of the primary refined liquid is lower than 15g/L, the primary refined liquid can be directly filtered by a nanofiltration membrane without large amount of water consumption for dilution. The primary refined liquid is filtered by the pressure of a nanofiltration device, the nanofiltration membrane has good interception effect on magnesium ions and sulfate radicals, and the interception rate is over 90 percent. The magnesium ions on the water production side of the nanofiltration device are lower than 1 g/L.

The concentration of lithium ions in the concentrated water side of the nanofiltration device is approximately equal to that of the primary refined liquid, and the concentration of magnesium ions is multiplied because of effective interception. In the nanofiltration process, the primary refined liquid is pressurized to 1.8MPa by a high-pressure pump and enters a nanofiltration membrane device, and the magnesium ions in the primary refined liquid are concentrated to three times. When the primary refined liquid passes through the nanofiltration device, the water yield is about 66.6 percent, and the water yield is the secondary refined liquid. The concentrated water rate is about 33.3 percent, and the concentrated water of the nanofiltration device is completely returned to the front section of the process (3) and diluted with lithium brine for preparation.

(5) The secondary refined liquid is subjected to ion exchange to form a tertiary refined liquid

In the secondary refined liquid, the concentration of lithium ions is about 11g/L, the concentration of magnesium ions is less than 1g/L, and the concentration of sulfate radicals is less than 0.03 g/L. The solution directly passes through chelating resin to carry out ion adsorption on calcium and magnesium ions. After the secondary refined liquid passes through the mixed bed, calcium and magnesium ions are lower than 20ppm, and a tertiary refined liquid is formed.

(6) The third refined liquid passes through a bipolar membrane electrodialyzer to obtain lithium hydroxide monohydrate solid and evaporation mother liquid

Feeding the three refined solutions into desalting chamber of bipolar membrane electrodialyzer, and under the action of electric field force, Li in the three refined solutions⁺Passes through the anode membrane to the base chamber and reacts with OH generated by the electrolysis of water through the bipolar membrane^-Pairing to form lithium hydroxide solution, refining X in the solution for three times^-Passes through the vaginal membrane to the acid compartment and reacts with H generated by the electrolysis of water through the bipolar membrane⁺Pairing to form HX solution;

and directly using concentrated alkali liquor in the alkali liquor buffer tank in the alkali liquor balance buffer tank as the polar liquid to realize the stable circulating operation of the polar liquid in the polar chamber of the bipolar membrane electrodialyzer, and overflowing from the alkali liquor buffer tank to obtain the lithium hydroxide solution.

(7) Passing the lithium hydroxide solution through an evaporative crystallizer to obtain a lithium hydroxide monohydrate solid and an evaporative mother liquor

And forcibly evaporating and concentrating the lithium hydroxide solution obtained by overflowing through an evaporation crystallizer, continuously discharging a mixture of a lithium hydroxide monohydrate solid and an evaporation mother liquor from a salt discharge angle of the evaporation crystallizer, and filtering the mixture to respectively obtain the lithium hydroxide monohydrate solid and the evaporation mother liquor, wherein the evaporation mother liquor is the lithium hydroxide solution.

(8) And (3) recrystallizing the lithium hydroxide monohydrate solid through washing to form battery-grade lithium hydroxide and washing liquid, wherein the washing liquid is a lithium hydroxide solution.

Advantageous effects

(1) The invention has good operability;

(2) the magnesium-lithium separation technology of the invention is a process for efficiently extracting lithium hydroxide and lithium carbonate from brine with high magnesium-lithium ratio;

(3) secondary purification is not needed, and a battery-grade lithium carbonate product meeting the standard is directly obtained;

(4) simultaneously obtaining a refined boric acid product;

(5) the lithium carbonate produced by the process has low cost.

Drawings

In order to more clearly describe the technical solution of the present invention, the following brief description is provided with reference to the accompanying drawings. It should be understood that these drawings are merely illustrative of some specific embodiments of the present application for making cell-grade lithium hydroxide and lithium carbonate, and are not intended to be limiting.

FIG. 1 is a flow chart of a conventional process for extracting boron by acidification;

FIG. 2 is a schematic diagram of ion-selective electrodialysis for separating Mg and Li;

FIG. 3 is a process flow diagram of the present invention;

FIG. 4 is a schematic view of a bipolar membrane electrodialyzer of the present invention; and

FIG. 5 is a diagram showing the operation of the bipolar membrane electrodialyzer of the present invention.

Wherein, the reference numbers:

1: anode chamber

2: cathode chamber

3: acid chamber

4: desalting chamber

5: alkali chamber

Detailed Description

For a further understanding of the present invention, preferred embodiments of the present invention will be described below with reference to examples. The description is intended to be illustrative of the features and advantages of the invention, and should not be taken to limit the scope of the invention.

Example 1

Salt lake intercrystalline brine of a salt lake (Uyuni) of the ulivia u you li salt lake is taken as an example:

(1) formation of boron-lithium brine

Salt lake intercrystalline brine of a salt lake of the Boragia Uyou Li is extracted to carry out salt field evaporation concentration, and tail liquid (mainly magnesium chloride solution) after lithium is extracted in a workshop is used for carrying out magnesium supplement operation in the salt field in the evaporation process. After potassium is extracted, the brine is led into a stabilization tank to be evaporated and stabilized, and boron-lithium brine with stable components is formed. The concentration of magnesium ions and sulfate radicals was monitored with emphasis. In order to save investment, the stabilization tank is simultaneously used as a brine storage tank, and brine stable components are stored after potassium is extracted by means of adjusting evaporation capacity, supplementing water and the like according to conditions such as production plans, ambient temperature and sunshine and the like.

The ingredient table of the intercrystalline brine and the brine after stable evaporation is as follows:

wuyuni salt lake (Uyuni) intercrystalline brine component:

component (A)	Li⁺	K⁺	Na⁺	Mg²⁺	Ca²⁺	B³⁺	SO₄ ^2-	Cl^-	Specific gravity of
										g/L	0.96	21.42	108.36	18.90	0.50	0.64	20.16	233.10	1.26

The brine components after evaporation stabilization of Wuyuni salt lake (Uyuni):

component (A)	Li⁺	K⁺	Na⁺	Mg²⁺	Ca²⁺	B³⁺	SO₄ ^2-	Cl^-	Specific gravity of
										g/L	6.63	0.53	1.21	112.22	0.33	8.64	23.17	346.12	1.37

(2) Extracting boron from boron-lithium brine

Filtering the boron-lithium brine to remove entrained solids, heating to 35-70 ℃, then adding the boron-lithium brine and 25-30% of industrial hydrochloric acid into a high-efficiency reactor in a certain proportion, fully mixing and reacting in the reactor, acidifying boron-containing substances in the boron-lithium brine, converting the boron-containing substances into boric acid, and forming boric acid crystals in the solution.

The acidified reaction solution was cooled to 15 ℃ and then filtered. Wherein 60% of the boric acid is collected by filtration and 40% of the boric acid is dissolved in the acidified solution. Filtering to obtain crude boric acid filtrate and acidified solution.

The acidified solution after cooling and filtration had a boron concentration of about 3g/L and a pH of 1.

And (3) passing the acidified solution through a centrifugal extractor to obtain lithium brine with boron concentration of less than 0.3g/L and lithium ion concentration of more than 5 g/L. The load extractant generated in the centrifugal extraction process and fresh water enter a centrifugal back-extraction machine to be mixed and subjected to back extraction, the generated heavy phase is boric acid solution, and the boron content in the boric acid solution is 9 g/L; the generated light phase is a no-load extractant which returns to the centrifugal extractor for recycling. Heating and dissolving the boric acid solution and the crude boric acid to form a boric acid saturated solution, filtering, and cooling and crystallizing to obtain a refined boric acid product with the purity of more than 99%.

(3) Electrodialytic ion-selective separation

Diluting the lithium brine by using concentrated water generated in the back-stage nanofiltration process, then feeding the diluted concentrated water into the dilution side of electrodialysis equipment provided with an ion selective membrane, and feeding the fresh water into the concentration side of the electrodialysis equipment. The ion selective membrane is preferably K-192 and ACS-8T membranes available from ASTOM. In the dilute side of the lithium bittern, monovalent ions pass through the ion selective membrane under the action of electric field force to reach the concentrated side, and a primary refined liquid is formed on the concentrated side. In the primary purified liquid, the lithium ion concentration was 12g/L and the magnesium ion concentration was 8 g/L. In the diluted solution, the concentration of lithium ions was 0.2g/L and the concentration of magnesium ions was 60 g/L.

(4) Filtering the primary refined liquid with nanofiltration membrane to obtain secondary refined liquid

Pressurizing the primary refined liquid to 2MPa by a high-pressure pump, and then entering a nanofiltration membrane device. The nanofiltration membrane is preferably a DK series membrane from GE. Controlling the flow of the concentrated water outlet of the nanofiltration equipment, and performing concentration operation by three times to form two parts of produced water and one part of concentrated water. The concentration of lithium ions in the concentrated water is close to that in the produced water, and both the concentration and the concentration are about 12 g/L. And the concentration of magnesium ions in the concentrated water is 24g/L, and the concentrated water is discharged to a lithium brine pool for dilution. The concentration of magnesium ions in the produced water is 0.3g/L, and the concentration of sulfate ions in the produced water is 0.06 g/L. The produced water is the secondary refined liquid.

(5) Ion exchange adsorption

And (4) passing the secondary refined liquid through a chelating resin mixed bed for ion exchange adsorption. The preferred adsorption resin is LSC-500 chelating resin of the company Xiao Zhao in China. In the post-adsorption solution, the concentration of magnesium ions was 8ppm and the concentration of calcium ions was 2ppm, and the obtained post-adsorption solution was a lithium chloride solution.

(6) Bipolar membrane electrodialysis process

A2 mol/l lithium chloride solution was passed through a bipolar membrane electrodialyzer to form a 2mol/l hydrochloric acid solution and a 2mol/l lithium hydroxide solution. Wherein the hydrochloric acid solution is returned to the hydrochloric acid solution in the previous step (2), and the lithium hydroxide solution enters the MVR evaporative crystallization process.

(7) MVR (mechanical vapor recompression evaporator) evaporation crystallization process

The 2mol/l lithium hydroxide solution is passed through an MVR evaporator, and the crystalline form is concentrated in the evaporation chamber of the MVR evaporator to form lithium hydroxide monohydrate. Discharging the solid-liquid mixture of the lithium hydroxide monohydrate through a salt discharge angle at the bottom of the evaporation chamber, and performing centrifugal filtration to form a lithium hydroxide monohydrate filtrate and a lithium hydroxide mother liquor. The lithium hydroxide mother liquor is used in the back-end process to produce battery grade lithium carbonate.

(8) Washing recrystallization of lithium hydroxide monohydrate

Adding lithium hydroxide monohydrate filtrate into a heating reaction kettle, adding quantitative deionized water, heating to dissolve, stirring and washing, then cooling and recrystallizing through a crystallizer, and filtering to form refined lithium hydroxide monohydrate and lithium hydroxide washing liquor. And drying and packaging the refined lithium hydroxide monohydrate to form the battery-grade lithium hydroxide. The lithium hydroxide wash solution is used in the back-end process to produce battery grade lithium carbonate.

(9) Carbonizing lithium hydroxide solution to form lithium carbonate

Mixing lithium hydroxide mother liquor formed in the MVR evaporation crystallization process with lithium hydroxide washing liquor to form a lithium hydroxide solution, enabling the lithium hydroxide solution and carbon dioxide gas to pass through a gas-liquid reactor to form a lithium carbonate crystal, and performing centrifugal filtration to form a lithium carbonate filtrate and tail liquor. And washing, drying and packaging the lithium carbonate filtrate to form the battery-grade lithium carbonate. And the tail liquid enters a comprehensive utilization process.

Example 2

Taking argentina lincoln salt lake (Rincon) intercrystalline brine as an example:

(1) collecting intercrystalline brine of Argentine lincoln salt lake, evaporating and concentrating to obtain boron-lithium brine components before large amount of bischofite is separated out. The brine can be further concentrated to a lithium ion concentration of 6.42g/L and a boron content of 4.65 g/L. However, the concentration process causes a great deal of lithium loss precipitated from bischofite, and simultaneously, a great deal of lithium sulfate is settled, and the total lithium ion loss exceeds 30 percent.

The composition of the lincoln salt lake (Rincon) intercrystalline brine:

component (A)	Li⁺	K⁺	Na⁺	Mg²⁺	Ca²⁺	B³⁺	SO₄ ^2-	Cl^-	Specific gravity of
										g/L	0.40	7.51	115.95	3.42	0.49	0.33	12.52	188.43	1.204

Brine components after further concentration in lincoln salt lake (Rincon):

component (A)	Li⁺	K⁺	Na⁺	Mg²⁺	Ca²⁺	B³⁺	SO₄ ^2-	Cl^-	Specific gravity of
										g/L	5.02	0.52	1.12	115.76	0.24	3.31	30.56	338.27	1.32

(2) Adjusting the pH value of the boron-lithium brine to 1 by using 20% hydrochloric acid to form an acidified solution, and passing the acidified solution and a no-load extractant through a centrifugal extractor to obtain the lithium brine with the heavy phase of which the boron concentration is lower than 0.4g/L and the lithium ion concentration is higher than 4.5 g/L. And (3) feeding the load extractant and fresh water into a centrifugal back-extraction machine for mixed back extraction, wherein the heavy phase is a boric acid solution, and the boron content in the boric acid solution is 7 g/L.

The light phase is a no-load extractant which returns to the centrifugal extractor for recycling. Because the boric acid solution formed by the back extraction has few impurities, the boric acid solution is evaporated by a forced evaporator to form 99.5 percent of high-purity boric acid which is separated out, and fresh water is recovered.

(3) Electrodialytic ion-selective separation

Diluting the lithium brine by using concentrated water generated in the back-stage nanofiltration process, then feeding the diluted concentrated water into the dilution side of electrodialysis equipment provided with an ion selective membrane, and feeding fresh water into the concentration side of the electrodialysis equipment. The ion selective membrane is preferably K-192 and ACS-8T ion selective membranes available from ASTOM. In the dilute side of the lithium bittern, monovalent ions pass through the ion selective membrane under the action of electric field force to reach the concentrated side, and a primary refined liquid is formed on the concentrated side. In the primary purified liquid, the lithium ion concentration was 11g/L and the magnesium ion concentration was 7 g/L. In the diluted solution, the concentration of lithium ions was 0.2g/L and the concentration of magnesium ions was 60 g/L.

Pressurizing the primary refined liquid to 2.5MPa by a high-pressure pump, and then feeding the primary refined liquid into a nanofiltration membrane device, wherein the nanofiltration membrane is preferably a DK series membrane of GE company. Controlling the flow of a concentrated water outlet of the nanofiltration equipment, and carrying out concentration operation by 3 times. The concentration of lithium ions in the concentrated water is close to that of lithium ions in the produced water, and the concentration of the lithium ions in the concentrated water is about 11 g/L.

The concentration of magnesium ions in the concentrated water is 21g/L, and the concentrated water is discharged to a lithium halide pool to form a dilution. The concentration of magnesium ions in the produced water is 0.3g/L, and the concentration of sulfate ions in the produced water is 0.06 g/L. The produced water is the secondary refined liquid.

(5) Ion exchange adsorption

And (4) passing the secondary refined liquid through a chelating resin mixed bed for ion exchange adsorption. The preferred adsorbent resin is LSC-500 chelating resin from Dawn company. In the obtained post-adsorption solution, the magnesium ion concentration was 8ppm and the calcium ion concentration was 2 ppm.

(6) Bipolar membrane electrodialysis process

(8) Washing recrystallization of lithium hydroxide monohydrate

(9) Carbonizing lithium hydroxide solution to form lithium carbonate

The above description of the embodiments is only intended to facilitate the understanding of the core ideas of the present invention. It should be noted that various changes and modifications could be made by those skilled in the art without departing from the principle of the invention, and these changes and modifications also fall into the scope of the invention as claimed.

Claims

1. A method for directly preparing lithium hydroxide and lithium carbonate from salt lake brine with high magnesium-lithium ratio comprises the following process steps:

(1) the brine after the potassium is extracted from the salt pan is naturally evaporated in a stabilizing pool by utilizing solar energy to reduce the total concentration of sodium and potassium ions in the brine, a potassium fertilizer raw material is generated in an adjusting pool, and concentrated boron and lithium brine with stable components is formed through natural evaporation and concentration;

(2) roughly filtering the boron-lithium brine, mixing the boron-lithium brine with industrial hydrochloric acid in a high-efficiency reactor for acidification reaction, cooling and filtering to form crude boric acid solid and acidified liquid, enabling the acidified liquid and a no-load extractant to enter a high-efficiency centrifugal extractor, extracting and deborizing the acidified liquid to form lithium brine, enabling a loaded extractant and fresh water to form a boric acid solution through a high-efficiency centrifugal back-extraction process, heating and dissolving the boric acid solution and the crude boric acid, and cooling and crystallizing to obtain refined boric acid;

(5) removing calcium, magnesium and sulfate radicals from the secondary refined liquid by a calcium and magnesium ion selective cation resin and a divalent anion selective resin ion exchanger to obtain a tertiary refined liquid, wherein the concentration of the calcium, magnesium ions and sulfate radicals is lower than 20ppm, the tertiary refined liquid is a feed liquid and is expressed by LiX, and X represents anions of the tertiary refined liquid;

(6) feeding the three refined solutions into desalting chamber of bipolar membrane electrodialyzer, and under the action of electric field force, Li in the three refined solutions⁺Passes through the anode membrane to the base chamber and reacts with OH generated by the electrolysis of water through the bipolar membrane^-Pairing to form lithium hydroxide solution, refining X in the solution for three times^-Passes through the vaginal membrane to the acid compartment and reacts with H generated by the electrolysis of water through the bipolar membrane⁺Pairing to form HX solution;

during the continuous operation of the bipolar membrane electrodialyzer, four solutions are respectively circulated in the bipolar membrane electrodialyzer, wherein the four solutions are respectively: feed liquid, acid liquid, alkali liquid and polar liquid,

wherein the balance buffer groove is designed by connecting double grooves in parallel, the double grooves are a balance groove and a buffer groove, the balance groove and the buffer groove are communicated through an opening at the bottom of a groove clapboard,

the top of the buffer tank is a circulating liquid inlet, and an overflow outlet is arranged at the liquid level position at the top of the buffer tank;

2. The method of claim 1, wherein: in the step (1), the brine after potassium extraction is further concentrated in a stabilization tank to form low-potassium-sodium boron-lithium brine, wherein the total amount of potassium and sodium ions is less than 5g/L, the product of the lithium ion concentration and the sulfate radical concentration is less than 150g²/L²。

3. The method of claim 1, wherein: in the step (2), the boron-lithium brine and hydrochloric acid are subjected to acidification reaction in one step through a high-efficiency reactor to generate boric acid, and crude boric acid is obtained through filtration, wherein the pH value of the mixed solution is 1-4.

4. The method of claim 1, wherein: in the step (2), the acidified solution and the no-load extractant pass through a centrifugal extractor, boric acid in the acidified solution is extracted to form lithium brine, the boric acid content in the lithium brine is lower than 0.6g/L, the load extractant and fresh water form a boric acid solution after passing through the centrifugal extractor, and the load extractant is subjected to boron removal through the centrifugal extractor to form the no-load extractant and is reused.

5. The method of claim 1, wherein: in the step (2), the boric acid solution formed by the back extraction and the crude boric acid formed by the acidification reaction of the boron-lithium brine are heated and dissolved to form a saturated boric acid solution, the temperature of the saturated boric acid solution is 30-80 ℃, and the refined boric acid is obtained by a cooling crystallization process.

6. The method of claim 1, wherein: in the step (3), the lithium brine passes through an electrodialyzer equipped with a monovalent ion selection membrane, monovalent ions in the lithium-rich brine permeate the ion selection membrane under the action of an electric field force, divalent ions in the lithium brine are intercepted by the ion selection membrane, and the process simultaneously completes magnesium-lithium separation and realizes lithium concentration to form primary refined liquid with high lithium and low magnesium.

7. The method of claim 1, wherein: and (4) selecting a nanofiltration membrane with a calcium-magnesium ion barrier effect, enabling the primary refined liquid to enter a nanofiltration device, further reducing the magnesium-lithium ratio, and obtaining secondary refined liquid, wherein the concentration of magnesium ions on the concentrated water side of the nanofiltration membrane is lower than 35g/L for the primary refined liquid entering the nanofiltration membrane process device.

8. The process according to claim 1, wherein the bipolar membrane electrodialyzer is a membrane module in the form of three compartments of a cation exchange membrane, an alkali compartment, a bipolar membrane, an acid compartment, an anion exchange membrane, a desalting compartment and a cation exchange membrane in combination between an anode compartment and a cathode compartment,

9. The process according to claim 1, wherein an anode plate is provided in an anode chamber and a cathode plate is provided in a cathode chamber of the bipolar membrane electrodialyzer,

10. The process according to claim 1, wherein the feed liquid is fed into a feed liquid equilibrium tank, wherein the feed liquid is continuously fed from a top inlet of the feed liquid equilibrium tank, and is discharged from a bottom outlet of the feed liquid equilibrium tank, and the feed liquid is pressurized by a feed liquid pump into a desalting chamber of the bipolar membrane electrodialyzer,

11. The method according to claim 10, wherein the diluent overflowing from the feed buffer tank is continuously added from a top inlet of an acid solution equilibrium tank,

12. The method of claim 10, wherein deionized water is continuously added from the top inlet of the lye equilibrium tank,

13. The process according to claim 10, wherein the lithium hydroxide solution is pressurized from the bottom outlet of the lye buffer tank into the electrode compartment of the bipolar membrane electrodialyzer by the anolyte pump, and the anolyte is returned from the top inlet of the lye equilibrium tank, forming an anolyte circulation.

14. The method of claim 1, wherein in step (7), the lithium hydroxide solution is concentrated by evaporation through an evaporative crystallizer, a mixture of lithium hydroxide monohydrate solids and evaporative mother liquor is withdrawn from a salt rejection corner of the evaporative crystallizer,

15. The method according to claim 14, wherein the evaporation mother liquor generated by centrifugal filtration and the washing liquor generated by washing recrystallization are carbonized with carbon dioxide gas in a gas-liquid reactor, the reaction temperature is controlled at 25 ℃, the pH value at the end of the reaction is controlled at 12, and lithium carbonate precipitate is formed,

16. The method of claim 1, wherein the base layer of the special cation exchange membrane is made of a corrosion-resistant material.

17. The method of claim 16, wherein the corrosion resistant material is polytetrafluoroethylene.