Method for preparing electrolytic lithium chloride by recovering chlorine from lithium hydroxide

Abstract

The invention relates to the technical field of lithium chloride preparation, and provides a method for preparing electrolytic lithium chloride by recovering chlorine from lithium hydroxide, which comprises the following steps: S1, cooling chlorine generated after electrolyzing and melting lithium chloride, and then introducing lithium hydroxide slurry for absorption to obtain mixed feed liquid; after hydroxyl radicals in the mixed feed liquid are consumed completely, chlorine gas continues to be introduced until the color of the feed liquid is changed from yellow to colorless, and absorption completion liquid is obtained; s2, adjusting the absorption completion liquid to be acidic, adding ferrous chloride, and reacting to obtain a primary purified liquid; s3, carrying out evaporative crystallization on the primary purified liquid, centrifuging, separating to obtain a solid, and drying the solid to obtain a product lithium chloride; according to the method, the excessive time and the excessive end point of chlorine gas introduction are controlled, so that the absorption liquid is acidic and is used for decomposing hypochlorite, and compared with an additional hypochlorite removal process in the prior art, the method is high in economical efficiency and simpler in process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium chloride preparation, and particularly relates to a method for preparing electrolytic lithium chloride by recycling chlorine gas from lithium hydroxide. BACKGROUND

[0002] In the electrolysis process of preparing metal lithium by electrolyzing molten lithium chloride, lithium ions are reduced to metal lithium at the cathode, and chlorine ions are oxidized to generate chlorine gas at the anode; due to the strong corrosiveness and toxicity of chlorine gas, a complex gas collection and purification system is required.

[0003] At present, an alkali solution is usually used as an absorption liquid in the prior art, such as sodium hydroxide solution for absorbing chlorine gas. Specifically, a patent with publication number CN114984735A discloses a method for treating chlorine gas by absorbing it with sodium hydroxide solution. This method can realize the resource utilization of chlorine gas, but will generate by-products such as hypochlorite or chlorate. If the lithium chloride prepared by this method is returned to the electrolytic cell as an electrolysis raw material, the hypochlorite or chlorate in the raw material will affect the electrolysis process and cause the peeling of carbon rods. Therefore, the patent further discloses that the generation amount of hypochlorite is first controlled by controlling the reaction between chlorine gas and alkali solution, and the natural light is used for decomposing the hypochlorite, and finally hydrogen peroxide is added to decompose the remaining hypochlorite. However, the decomposition efficiency of natural light is low, and a large amount of hydrogen peroxide still needs to be used to achieve high hypochlorite treatment efficiency, but the cost of hydrogen peroxide is high, and the economic efficiency is low.

[0004] As for the treatment of hypochlorite, a patent with publication number CN110668474A discloses a method for preparing lithium chloride by using chlorine gas. Specifically, the patent adds ammonium chloride with small Gibbs free energy and easy reaction into the solution to convert most of the lithium hypochlorite in the solution according to theoretical calculation, and then adds hydrogen peroxide with high reaction limit to chlorinate again. However, the method for preparing lithium chloride by using ammonium chloride has the defect of low purity, and hydrogen peroxide still needs to be further used for purification. More importantly, the method needs to be calculated theoretically according to different production and application conditions to accurately control the proportion of ammonium chloride and hydrogen peroxide, as well as the reaction time, addition sequence and addition timing, which makes it difficult to control the final purity of the product.

[0005] Therefore, based on the above description, there is an urgent need for a method for preparing electrolytic lithium chloride by recycling chlorine gas from lithium hydroxide with low production cost and high product purity. SUMMARY

[0006] In view of the above deficiencies in the prior art, the purpose of the present application is to provide a method for preparing electrolytic lithium chloride by recycling lithium hydroxide chlorine, which controls the timing and excess end point of the chlorine gas inlet, and further makes the absorption liquid acidic to achieve the purpose of decomposing hypochlorite in large quantities, without using high-cost hydrogen peroxide and without the need for complex calculations and precise control to achieve high economic efficiency and simpler process.

[0007] The present application is realized by the following technical solutions: A method for preparing electrolytic lithium chloride by recycling lithium hydroxide chlorine, comprising the following steps: S1 cooling the chlorine gas generated after electrolytic molten lithium chloride, then absorbing it with lithium hydroxide slurry to obtain a mixed liquid; when the hydroxyl ions in the mixed liquid are consumed, continue to introduce chlorine gas until the color of the liquid changes from yellow to colorless to obtain an absorption completed liquid; S2 adjusting the absorption completed liquid to be acidic, then adding ferrous chloride, and obtaining a primary purified liquid after reaction; S3 evaporating and crystallizing the primary purified liquid, then separating the obtained solid by centrifugation, drying the solid, and obtaining product lithium chloride.

[0008] Technical mechanism The high-temperature chlorine gas generated by electrolytic molten lithium chloride is first cooled, then introduced into lithium hydroxide slurry, and then chlorine gas reacts with LiOH to generate a mixed solution of lithium chloride, lithium hypochlorite and lithium chlorate, etc. In this process, chlorine gas acts as an oxidizing agent and reacts with hydroxyl ions in LiOH to generate chloride ions, hypochlorite ions and chlorate ions, and the reaction equation is as follows: Cl2+2LiOH→LiCl+LiClO+H2O Cl2+6LiOH→5LiCl+LiClO3+3H2O When the OH - in the absorption liquid is completely consumed, the solution is acidic, and the continued introduction of chlorine gas will cause the decomposition of a large amount of hypochlorite, because hypochlorite is unstable under acidic conditions and is prone to disproportionation to generate chloride ions and oxygen, until the concentration of hypochlorite is less than 10g / L, the color of the liquid changes from yellow to colorless, which is determined as the absorption end point, and the introduction of chlorine gas is stopped to obtain the absorption completed liquid, and the reaction equation is as follows: 3ClO − →2Cl − +ClO3 − Further, by adding ferrous chloride, a redox reaction with chlorate ions can be carried out to reduce the chlorate ions into chloride ions, and at the same time, iron ions are generated. The iron ions do not affect the electrolysis and can not be removed, but they can also be removed by adjusting the pH value again to make the iron ions form a precipitate, and then the purified completed liquid is obtained after filtration. The reaction equation is as follows: 2FeCl2+ClO 3− +3H + →2Fe 3+ +Cl − +Cl2+H2O The purified completed liquid is evaporated and crystallized, and then the solid and the mother liquor are separated by centrifugation. The solid is further dried at 350-500°C. At this temperature, not only can the crystal water be separated, but also the residual chlorate can be completely decomposed. Finally, the anhydrous lithium chloride product for electrolysis is obtained. In addition, the mother liquor after separation is returned to the next batch of mixed solution for further absorption of chlorine, and this cycle is repeated. However, with multiple cycles, the chlorate will be enriched. Further, as mentioned above, the use of ferrous chloride as a reducing agent to react with chlorate in a strong acidic environment can effectively decompose the enriched chlorate, ensuring the purity of the lithium chloride product.

[0009] Further, in the selection of the absorption liquid, a high-concentration lithium hydroxide slurry is used. Since the saturation concentration of lithium chloride is 150-160, and the saturation concentration of lithium hydroxide is 70, and since lithium hydroxide exists in the form of monohydrate (LiOH・H2O), it can be prepared into a slurry system for absorption operation during the absorption process. Further, in the absorption process, the crystal water contained in the monohydrated lithium hydroxide is released, and when it is converted into lithium chloride, the solubility of lithium chloride in water is significantly higher than that of lithium hydroxide, which can further improve the solubility and absorption efficiency of the system.

[0010] Preferably, in step S1, the chlorine gas is cooled to below 40°C.

[0011] Preferably, in step S1, the chlorine gas is cooled to 20-40°C.

[0012] Preferably, in step S1, the concentration of hypochlorite in the absorption completed liquid is less than 10 g / L.

[0013] Preferably, in step S1, the concentration of hypochlorite in the absorption completed liquid is less than 2 g / L.

[0014] Preferably, in step S2, the molar ratio of ferrous chloride to chlorate is 2-8:1, and the reaction time is 1-1.5 h.

[0015] Preferably, in step S2, the pH value of the absorption completed liquid is adjusted to 1-5.

[0016] Preferably, in step S2, the pH value of the primary purification liquid is adjusted to 6-9, and the purified liquid is obtained by filtering after precipitation.

[0017] Preferably, in step S3, the drying temperature is 350-550 DEG C.

[0018] The drying time is specifically selected according to the drying temperature, and in general, the drying time is 0.5 h-3 h, which aims to decompose residual chlorate in the product.

[0019] The time varies from 3 h to 0.5 h according to the temperature, which aims to decompose residual chlorate in the product.

[0020] Preferably, the mother liquor is separated after centrifugation, and the mother liquor is mixed with the mixed material liquid in step S1.

[0021] The mother liquor separated after centrifugation needs to be regularly treated to remove impurities, specifically metal impurities and sulfate radicals, and then mixed with the mixed material liquid.

[0022] The beneficial effects of the present application are: (1) The present application controls the timing and excess end of the chlorine gas, so that the absorption liquid is acidic, which can effectively decompose a large amount of hypochlorite, and compared with the prior art, the present application does not use high-cost hydrogen peroxide, and does not need to perform complex calculation and accurate control, so that the economic efficiency is high, and the process is simpler.

[0023] (2) The high-concentration lithium hydroxide slurry is used as the absorption liquid, and the high OH - concentration can effectively improve the absorption rate, and the lithium concentration of the absorption completed liquid is higher, which can reduce the cost of subsequent evaporation and crystallization.

[0024] (3) The evaporation mother liquor after centrifugal separation is returned to the mixed material liquid to form a closed loop, and there is no waste liquid discharge; the enrichment of chlorate in the mixed material liquid is treated by using ferrous chloride, which can decompose chlorate without affecting the subsequent application of lithium chloride in electrolysis, and the generated iron ions can be easily precipitated and separated by adjusting the pH value, so that the impurity removal difficulty of lithium chloride is effectively reduced, and high-purity electrolytic lithium chloride is prepared.

[0025] (4) High-temperature drying can separate the crystal water in the product, and further decompose a small amount of residual chlorate and hypochlorite in the product, so as to further improve the quality of lithium chloride product, and when lithium chloride is reused as an electrolysis raw material, the content of chlorate is lower, and the electrode can be better protected. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some of the embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0027] Figure 1 The process flow chart of the present application. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely. The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions suggested by the manufacturer. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained by market purchase.

[0029] Embodiment 1 The present embodiment provides a method for preparing lithium chloride for electrolysis by recovering lithium hydroxide and chlorine, comprising the following steps: <1> Chlorine cooling and absorption reaction The high-temperature chlorine gas generated by electrolysis of molten lithium chloride is cooled to 40℃ by using a water-cooled heat exchanger, and then introduced into a lithium hydroxide slurry (in a suspended state) with a concentration of 150 g / L for absorption. During the absorption process, the change of the solution pH value is continuously monitored. When the hydroxide in the solution is completely consumed, the chlorine gas is continuously introduced, and the hypochlorite concentration is detected every 10 min during the period. When the pH value of the solution is reduced to 3-4, and the hypochlorite concentration is reduced to 10 g / L, and the color of the solution changes from yellow to colorless, it is determined that the absorption is completed, and the absorption completed liquid is obtained.

[0030] <2> Ferrous chloride to remove hypochlorite and secondary purification To the absorption completed liquid of step <1>, 30% hydrochloric acid is slowly added while stirring, and the pH value of the solution is adjusted to 1. Then, under stirring, ferrous chloride solution (concentration of 100 g / L) is slowly added, so that the molar ratio of ferrous chloride to chlorate is 2:1. The reaction is continued for 1 h to obtain a primary purification liquid. Then, sodium hydroxide solution is added to adjust the pH value of the primary purification liquid to 6. After standing for 30 min, filtration is carried out to obtain a purification completed liquid.

[0031] <3> Evaporation crystallization and preparation of lithium chloride product The purified solution obtained in step <2> is fed into an evaporation crystallizer, and evaporation crystallization is carried out under the conditions of a vacuum degree of -0.08 MPa and a temperature of 80°C. After crystallization is completed, lithium chloride wet product and mother liquor are obtained by centrifugal separation, and the mother liquor is fed into the mixing solution storage tank in step <1> for mixing. The lithium chloride wet product is placed in a vacuum drying oven and dried at 350°C under a vacuum degree of -0.09 MPa for 3 h to obtain lithium chloride product.

[0032] Example 2 The difference between this example and Example 1 is that: <1> Chlorine cooling and absorption reaction High-temperature chlorine gas generated by electrolytic molten lithium chloride is cooled to 35°C by a finned cooler and absorbed into lithium hydroxide slurry with a concentration of 200 g / L. The pH value of the solution is monitored in real time during the absorption process. When the hydroxide is consumed, chlorine is continuously introduced. The hypochlorite concentration is measured every 15 min. When the solution pH is between 3 and 4, the hypochlorite concentration is reduced to 5 g / L, and the solution color is colorless, the chlorine introduction is stopped, and the absorption completion solution is obtained.

[0033] <2> Ferrous chloride removal of chlorate and secondary purification Hydrochloric acid with a mass fraction of 30% is added dropwise to the absorption completion solution, and the pH value is adjusted to 3 under stirring. Then, ferrous chloride solution (concentration of 150 g / L) is slowly added under stirring, and the molar ratio of ferrous chloride to chlorate is controlled to be 6:1. After 1.25 h of reaction, a primary purification solution is obtained. Sodium hydroxide solution is added to adjust the pH of the primary purification solution to 7. After standing for 40 min, the solution is filtered to obtain a purification completion solution.

[0034] <3> Evaporation crystallization and lithium chloride product preparation The lithium chloride wet product is placed in a vacuum drying oven and dried at 400°C under a vacuum degree of -0.09 MPa for 2 h to obtain lithium chloride product.

[0035] Example 3 The difference between this example and Example 1 is that: <1> Chlorine cooling and absorption reaction High-temperature chlorine gas is cooled to 20°C by a tube cooler and absorbed into lithium hydroxide slurry with a concentration of 300 g / L. The pH value is monitored during the absorption process. When the hydroxide is consumed, chlorine is continuously introduced. The hypochlorite concentration is detected every 20 min. When the solution pH is 3-4, the hypochlorite concentration is reduced to 2 g / L, and the solution is colorless, the chlorine introduction is stopped, and the absorption completion solution is obtained.

[0036] <2> Ferrous chloride removal of chlorate and secondary purification The pH value of the absorption completed solution is adjusted to 5 by adding 30wt% hydrochloric acid. A ferrous chloride solution (concentration of 200g / L) is added under stirring, and the molar ratio of ferrous chloride to chlorate is controlled to be 8:1. The reaction is carried out for 1.5h to obtain a primary purified solution. The pH value is adjusted to 9 by adding sodium hydroxide solution. After standing for 50min, the precipitate is filtered to obtain a purified solution.

[0037] <3> Evaporative crystallization and preparation of lithium chloride product The wet lithium chloride product is placed in a vacuum drying oven, and dried at 500℃ and a vacuum degree of -0.09MPa for 0.5h to obtain a lithium chloride product.

[0038] Comparative Example 1 The difference between this comparative example and Example 1 is that: In Step <1>, the high-temperature chlorine gas generated by electrolytic molten lithium chloride is cooled to 40℃, and is absorbed into 150g / L lithium hydroxide slurry. When the hydroxyl groups in the solution are completely consumed, the chlorine gas is stopped (without controlling the excess chlorine gas), at which time the pH value of the solution is 7-8, and the hypochlorite concentration is 25g / L (without decomposition).

[0039] In Step <1>, 30wt% hydrogen peroxide is added to the absorption completed solution according to a molar ratio of hypochlorite to hydrogen peroxide of 1:0.008, and the reaction is carried out for 10min to remove the hypochlorite.

[0040] Comparative Example 2 The difference between this comparative example and Example 1 is that: In Step <1>, the chlorine gas is cooled to 40℃, and is absorbed into 50g / L low-concentration lithium hydroxide solution (non-slurry). The excess chlorine gas is controlled to have a pH value of 3-4.

[0041] Comparative Example 3 The difference between this comparative example and Example 1 is that: In Steps <1> to <3>, the mother liquor after centrifugal separation is directly discharged, and is not returned to the mixed solution.

[0042] In Step <4>, when the chlorate is enriched in the solution, ammonium chloride is used instead of ferrous chloride for treatment according to a molar ratio of hypochlorite to ammonium chloride of 1:0.01, and the remaining steps are unchanged.

[0043] Test Example The content of LiCl and various impurities in the lithium chloride product was determined by taking Examples 1-3 and Comparative Examples 1-3 as samples; wherein, the content of LiCl was tested according to GB / T11064.3-2013; the content of various impurities was tested according to: Na, K: GB / T11064.4-2013, SO4²⁻: GB / T11064.9-2013, Ca, Mg: GB / T11064.5-2013, GB / T11064.6-2013, Fe: GB / T11064.7-2013, ClO⁻: iodimetry, NH4 + The results are shown in Table 1.

[0044] Table 1 Test results of samples

[0045] As can be seen from Table 1, in Examples 1-3, by prolonging the chlorine gas passing time (until the solution is colorless), making the absorption solution acidic (pH 3-4), promoting the disproportionation reaction of hypochlorite, reducing the residual ClO⁻, and avoiding its dilution of the main content; the concentration of lithium hydroxide slurry is increased from 150 g / L (Example 1) to 300 g / L (Example 3), taking advantage of the high solubility of lithium chloride (saturation concentration 150-160 g / L), to increase the lithium concentration of the absorption completion solution (Li2O 70-80 g / L), and reduce the relative proportion of impurities in the evaporation process; the molar ratio of ferrous chloride to chlorate is increased from 2:1 (Example 1) to 8:1 (Example 3), which completely removes ClO3⁻ through redox reaction and reduces impurity residues; the drying temperature is increased from 350°C to 500°C, which not only removes crystal water, but also decomposes residual chlorate, further improving the purity; Fe³⁺ generates Fe(OH)3 precipitate by adjusting the pH to 6-9 (Examples 1-3 are adjusted to 6, 7, and 9, respectively), and the residual Fe is ≤0.0016% after filtration, which does not affect the electrolytic application; hydrogen peroxide (Comparative Example 1) or ammonium chloride (Comparative Example 3) is not used, which avoids the introduction of ClO⁻ or NH4⁺, and ensures the applicability of the product.

[0046] Comparative Example 1 does not control the excess of chlorine, relies on hydrogen peroxide to remove hypochlorite, and causes residual impurities; Comparative Example 2 uses a low-concentration absorption solution, which has low absorption efficiency and impurity enrichment; Comparative Example 3 breaks the mother liquor closed loop and uses ammonium chloride to remove impurities, which causes lithium loss and the introduction of new impurities, ultimately resulting in low main content (92.3%-94.1%) and high impurity residues in Comparative Examples, which cannot achieve the technical effects of Examples.

[0047] The above is only a preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included within the protection scope of the present application.

Claims

1. A process for the production of electrolytic lithium chloride from lithium hydroxide recovered chlorine, characterized in that: The method comprises the following steps: S1: cooling chlorine gas generated after electrolytic molten lithium chloride, and then introducing the lithium hydroxide slurry into the chlorine gas for absorption to obtain a mixed solution; when the hydroxyl ions in the mixed solution are consumed, continuously introducing chlorine gas until the color of the solution changes from yellow to colorless to obtain an absorption completed solution; S2: adjusting the absorption completed solution to be acidic, and then adding ferrous chloride to obtain a first purified solution after reaction; S3: evaporating and crystallizing the first purified solution, separating the obtained solid after centrifugation, drying the solid, and obtaining product lithium chloride.

2. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to claim 1, characterized in that: In step S1, the chlorine gas is cooled to below 40℃.

3. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to claim 1, characterized in that: In step S1, the concentration of the lithium hydroxide slurry is 100-400g / L.

4. The process for the recovery of chlorine from lithium hydroxide to produce electrolytic lithium chloride according to claim 1, characterized in that: In step S1, the concentration of hypochlorite in the absorption completed solution is less than 10g / L.

5. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to claim 4, characterized in that: In step S1, the concentration of hypochlorite in the absorption completed solution is less than 2g / L.

6. The process for the recovery of chlorine from lithium hydroxide to produce electrolytic lithium chloride according to claim 1, characterized in that: In step S2, the molar ratio of ferrous chloride to chlorate is 2-8:1, and the reaction time is 1-1.5h.

7. The method of claim 1, wherein the lithium hydroxide is recovered from the chlorine gas produced by the electrolysis of lithium chloride. In step S2, the pH value of the absorption completed solution is adjusted to 1-5.

8. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to claim 7, characterized in that: In step S2, the pH value of the first purified solution is adjusted to 6-9, and the purified solution is obtained after filtration after precipitation.

9. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to claim 1, characterized in that: In step S3, the drying temperature is 350-550℃.

10. The process for the production of electrolytic lithium chloride by recovery of chlorine from lithium hydroxide according to any one of claims 1 to 9, characterized in that: In step S3, the mother liquor is also separated after centrifugation, and the mixed solution in step S1 is mixed with the mother liquor.

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