A bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate

Abstract

This invention provides a bipolar membrane electrodialysis system and method for preparing lithium hydroxide and citric acid using lithium citrate as a raw material. The system utilizes a laboratory-assembled two-compartment BMED membrane stack electrodialysis device, arranged in a bipolar membrane-cation exchange membrane-bipolar membrane configuration, divided into an alkali compartment and a salt compartment. A 0.5 mol / L lithium citrate solution is added to the salt compartment of the bipolar membrane electrolyzer, a 0.3 mol / L sodium sulfate solution is added to the electrode compartment, and a pure aqueous solution is added to the alkali compartment. A direct current is then applied to the bipolar membrane electrolyzer at a current density of 30-80 mA / cm². 2 Then, electrolysis begins. A lithium hydroxide solution is obtained at the outlet of the alkali chamber, and a citric acid solution is obtained at the outlet of the salt chamber. Compared with other processes, this invention generates no solid waste, has a simple operation, short processing time, low economic cost, and is environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of membrane electrolysis, ion exchange, and lithium ion recovery, and specifically relates to a method for producing citric acid and lithium hydroxide from an organic lithium salt solution via bipolar membrane electrodialysis. Background Technology

[0002] With the rapid growth in the production and sales of new energy vehicles and the development of the energy storage industry, the demand for lithium for batteries is rising rapidly. Global lithium demand is approximately 860,000 tons (LCE) per year, an increase of 53.7% year-on-year. Lithium compounds are widely used in industry, such as in the production of glass, ceramics, air conditioning refrigerants, and lithium batteries, playing an important role in energy storage, production capacity, and energy conservation. While my country is a major lithium resource country, its lithium extraction technology is relatively backward, mainly due to high energy consumption, environmental unfriendliness, and low purity of the obtained lithium hydroxide. Therefore, new, environmentally friendly production processes capable of producing high-purity lithium hydroxide products are urgently needed.

[0003] For many years, numerous research institutions and enterprises have devoted considerable effort to researching the problems encountered in the production of lithium hydroxide. Among these efforts, bipolar membrane electrodialysis technology has seen significant promotion and application. In the process of recovering lithium hydroxide from solution using the bipolar membrane method, the lithium sulfate solution is treated with a bipolar membrane system, and the products after treatment are lithium hydroxide and sulfuric acid. Bipolar membrane technology has also been used to convert lithium carbonate from salt lake brine to lithium hydroxide and lithium phosphate. A bipolar membrane is an ion exchange membrane with special functions; under the influence of an electric field, its middle layer undergoes water dissociation, producing H₂O. + and OH - Ions. Bipolar membrane electrodialysis technology combines this special function with ordinary electrodialysis, thereby enabling the immediate production / regeneration of acids / alkalis, or acidification and / or alkalization.

[0004] The rapid industrial development in recent years has led to an increase in the emission of inorganic salts and organic acid salts, causing not only environmental pollution but also wasting a large amount of useful resources. Currently, most research on bipolar membrane electrodialysis technology focuses on its application in inorganic acid salts. This invention primarily studies the application and impact of bipolar membrane electrodialysis on organic acid salts. Lithium citrate, an organic acid salt, is generally synthesized from citric acid and lithium carbonate. It is readily soluble in water and has wide applications in pharmaceuticals, construction, food, analytical chemistry, and lithium battery recycling. Citric acid, one of its raw materials, is one of the world's largest organic acids and can be used as a green and environmentally friendly leaching agent for waste lithium-ion batteries. It is a naturally occurring organic compound found in citrus fruits. Its production methods mainly include fruit extraction, chemical synthesis, and microbial fermentation, resulting in a relatively long process flow.

[0005] This invention addresses the aforementioned problems by proposing a bipolar membrane electrolysis process for preparing lithium hydroxide and citric acid from lithium citrate. A compact, easy-to-install, and simple-to-operate electrodialysis membrane stack is rationally designed for this process. This device is simple, lightweight, and can efficiently and stably electrolyze lithium citrate salt solutions to obtain high-concentration lithium hydroxide and citric acid. Summary of the Invention

[0006] The purpose of this invention is to provide a bipolar membrane electrodialysis process for preparing lithium hydroxide and citric acid using lithium citrate as a raw material, comprising the following steps: using a two-compartment bipolar membrane electrodialysis structure, a laboratory-prepared lithium citrate solution is fed into a bipolar membrane electrodialysis device for electrodialysis ion exchange treatment to obtain citric acid and lithium hydroxide solutions.

[0007] This invention utilizes bipolar membrane electrodialysis to produce citric acid and lithium hydroxide. The specific steps of this process are as follows:

[0008] First, a lithium citrate solution (approximately 0.5 mol / L) is introduced into the salt chamber of the bipolar membrane electrodialysis stack, pure water is introduced into the alkali chamber, and a strong electrolyte solution (0.3 mol / L sodium sulfate solution) is introduced into the cathode and anode chambers, respectively. The feed solution in each chamber is then circulated using a peristaltic pump for 30 minutes to remove air bubbles from the membrane stack.

[0009] In a further preferred embodiment, the effective area of ​​both the cation exchange membrane and the bipolar membrane in the bipolar membrane electrodialysis is 9 cm². 2 The process employs constant current operation, applying direct current (current density of 30-80 mA / cm²) across the bipolar membrane electrodialysis stack. 2 Under the influence of direct current, hydrogen ions generated by the hydrolysis of the bipolar membrane combine with citrate ions separated from the salt chamber to form citric acid, while hydroxide ions generated by the hydrolysis of the bipolar membrane combine with lithium ions migrating from the salt chamber to the alkali chamber to form lithium hydroxide. Therefore, a lithium hydroxide solution can be obtained in the alkali chamber and a citric acid solution can be obtained in the salt chamber.

[0010] More preferably, during the electrodialysis process, the flow rate of the solutions in the anode chamber, cathode chamber, alkali chamber, and salt chamber is controlled at 25 mL / min by a peristaltic pump.

[0011] The apparatus used in the process of this invention adopts the following technical solution:

[0012] The two-compartment bipolar membrane electrodialysis device includes a laboratory-assembled BMED membrane stack, such as Figure 1As shown, the bipolar membrane electrodialysis device includes a bipolar membrane stack, a feed tank, a power supply, and a peristaltic pump. The BMED membrane stack uses a custom-designed acrylic plate as the frame for the alkali and salt chambers. Both side plates are ruthenium-coated titanium plates, and both are flat plates. The device's internal dimensions are 3cm in length and 3cm in height, with an effective membrane area of ​​9cm². 2 The thickness of the alkali chamber, salt chamber, and electrode chamber is 1.5 cm, and adjacent chambers are sealed with 2 mm thick rubber strips. Figure 2 As shown, the bipolar membrane electrodialysis membrane stack is composed of bipolar membrane (BPM1 in the figure), cation exchange membrane (CEM in the figure), and bipolar membrane (BPM2 in the figure) stacked alternately, plus a flow channel mesh and a sealing gasket.

[0013] Preferably, the two-compartment electrodialysis includes an alkali compartment and a salt compartment, arranged as follows: anode-bipolar membrane-cation exchange membrane-bipolar membrane-cation exchange membrane-bipolar membrane-cathode. The cathode side of the bipolar membrane, together with the cation exchange membrane, forms the alkali compartment, and the anode side of the bipolar membrane, together with the cation exchange membrane, forms the salt compartment.

[0014] Preferably, the anode chamber and cathode chamber are connected in parallel to the electrolyte storage tank, the alkali chamber is connected to the alkali solution tank, and the salt chamber is connected to the feed tank. The electrolyte storage tank forms a circulation loop between the anode and cathode chambers via electrolyte peristaltic pumps, the alkali solution tank and alkali chamber form a circulation loop via an alkali peristaltic pump, and the feed tank and salt chamber form a circulation loop via a salt chamber peristaltic pump. The anode and cathode chambers are connected to the positive and negative electrodes, respectively.

[0015] Preferably, the alkali chamber outlet is connected to the inside of the alkali tank via a silicone tube, and the alkali chamber inlet is connected to the outlet of the alkali chamber peristaltic pump via a silicone tube; the feed liquid outlet is connected to the inside of the feed liquid tank via a silicone tube, and the feed liquid inlet is connected to the outlet of the feed chamber peristaltic pump via a silicone tube; the electrolyte outlet is connected to the inside of the electrolyte storage tank via a silicone tube, and the electrolyte inlets are connected to the outlets of the anode chamber and cathode chamber peristaltic pumps via silicone tubes respectively.

[0016] More preferably, both the cation exchange membrane and the bipolar membrane are homogeneous ion exchange membranes, purchased from Hangzhou Huamo Technology Co., Ltd.

[0017] This invention employs a bipolar membrane electrodialysis process to separate lithium citrate to prepare lithium hydroxide and citric acid. Compared with traditional causticization and electrolysis methods, the process is simpler and shorter. No solid waste, waste acid, or waste alkali is generated during electrodialysis, and large amounts of water are also avoided. The raw materials exhibit good atom economy. The bipolar membrane electrodialysis method can produce high-purity lithium hydroxide with few impurities, resulting in high lithium resource utilization, minimal waste, and environmental friendliness. Furthermore, the citric acid produced in the salt chamber can be recycled back into the lithium production process, saving production costs. Attached Figure Description

[0018] Appendix Figure 1 This is a schematic diagram of the bipolar membrane electrodialysis experimental apparatus of the present invention.

[0019] Appendix Figure 2 This is a schematic diagram of the two-compartment bipolar membrane electrodialysis membrane stack structure according to the present invention. Detailed Implementation

[0020] The preparation of lithium hydroxide and citric acid from lithium citrate by bipolar membrane electrodialysis according to the present invention will be described in detail below with reference to the embodiments and accompanying drawings. The technical solutions in the embodiments of this application will be clearly and completely described. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The present invention is not limited to the following embodiments.

[0021] Example 1:

[0022] Step 1: Add 100 ml of 0.5 mol / L lithium citrate solution to the salt chamber of the bipolar membrane electrolyzer, add 200 ml of 0.3 mol / L sodium sulfate solution to the cathode and anode chambers of the bipolar membrane electrolyzer, and add 100 ml of pure water to the alkali chamber of the bipolar membrane electrolyzer.

[0023] Step 2: Start the peristaltic pump and circulate the solution in the electrodialysis device for 30 minutes. Set the peristaltic pump flow rate to 25 ml / min.

[0024] Step 3: Apply DC current to the bipolar membrane electrolyzer and adjust the voltage of the bipolar membrane electrolyzer to the maximum power supply voltage of 60V, and adjust the current density of the electrolyzer to 70mA / cm². 2 Then electrolysis begins.

[0025] Step 4: Under the influence of the electric field, lithium ions in the salt chamber gradually move towards the ion exchange membrane and pass through the cation exchange membrane, migrating from the salt chamber to the alkali chamber. The lithium ions entering the alkali chamber combine with hydroxide ions to form lithium hydroxide. The lithium hydroxide solution flows out from the outlet of the alkali chamber at a concentration of 0.83 mol / L. Meanwhile, citric acid solution with a concentration of 1.25 mol / L flows out from the outlet of the salt chamber of the bipolar membrane electrolyzer. The current efficiency is 28.48%, and the energy consumption is 35.32 kWh / kg.

[0026] Example 2:

[0027] Referring to Example 1, in steps 1-2), a direct current is supplied to the bipolar membrane electrolyzer, and the voltage of the bipolar membrane electrolyzer is adjusted to the maximum power supply voltage of 60V, and the current density is adjusted to 40mA / cm². 2The experiment was conducted as follows. Lithium hydroxide solution with a concentration of 0.7 mol / L flowed out from the outlet of the alkali chamber; while citric acid solution with a concentration of 0.82 mol / L flowed out from the outlet of the salt chamber of the bipolar membrane electrolyzer. The current efficiency was 40.08%, and the energy consumption was 16.45 kWh / kg.

[0028] Example 3:

[0029] Referring to Example 1, in steps 1-2), a direct current is supplied to the bipolar membrane electrolyzer, and the voltage of the bipolar membrane electrolyzer is adjusted to the maximum power supply voltage of 60V, and the current density is adjusted to 60mA / cm². 2 The experiment was conducted as follows. Lithium hydroxide solution with a concentration of 0.81 mol / L flowed out from the outlet of the alkali chamber; while citric acid solution with a concentration of 1.28 mol / L flowed out from the outlet of the salt chamber of the bipolar membrane electrolyzer. The current efficiency was 30.15%, and the energy consumption was 28.26 kWh / kg.

[0030] Example 4:

[0031] Referring to Example 1, in steps 1-2), a direct current is supplied to the bipolar membrane electrolyzer, and the voltage of the bipolar membrane electrolyzer is adjusted to the maximum power supply voltage of 60V, and the current density is adjusted to 80mA / cm². 2 The experiment was conducted as follows. Lithium hydroxide solution with a concentration of 0.84 mol / L flowed out from the outlet of the alkali chamber; while citric acid solution with a concentration of 1.15 mol / L flowed out from the outlet of the salt chamber of the bipolar membrane electrolyzer. The current efficiency was 23.45%, and the energy consumption was 46.11 kWh / kg.

[0032] It can be seen that the higher the current density in the bipolar membrane electrodialysis device, the higher the lithium hydroxide concentration in the alkali chamber and the higher the lithium citrate concentration in the acid chamber. When the current density is too high, it is easy to cause ion competition and ion leakage, resulting in loss of current efficiency.

[0033] In this invention, the concentrations of the lithium hydroxide solution and citric acid solution obtained by bipolar membrane electrodialysis are determined by acid-base titration using phenolphthalein solution. During the experiment, 1 ml samples are taken from the alkali and acid compartments every 1 hour using a pipette for analysis.

[0034] In this invention, lithium ion detection is performed using an inductively coupled plasma spectrometer (ICP).

[0035] The formula for calculating the energy consumption (E) of a bipolar membrane electrodialysis system for producing lithium hydroxide is as follows (1):

[0036]

[0037] Where C tU is the concentration of the lithium hydroxide solution in the alkaline tank; U is the membrane stack voltage drop at time t; I is the current used; M is the molecular weight of lithium hydroxide; and V is the initial volume of the solution in the alkaline tank.

[0038] The formula for calculating the current efficiency (η) of a bipolar membrane electrodialysis system for producing lithium hydroxide is as follows (2):

[0039]

[0040] Where C0 is the initial concentration of the lithium hydroxide solution, F is the Faraday constant, and N is the number of repeating units (N=1).

[0041] The embodiments described above are merely examples to clearly illustrate the present invention and do not limit the implementation of the invention. The above embodiments and descriptions only illustrate the principles of the invention, and those skilled in the art can make other variations and improvements based on the present invention. It is impossible to describe all embodiments here. All obvious modifications based on the technical solutions derived from the present invention are within the protection scope of the present invention.

Claims

1. A bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate, characterized in that: The two-compartment bipolar membrane electrodialysis device includes a laboratory-assembled BMED membrane stack, a feed tank, a power supply, and a peristaltic pump. The BMED membrane stack uses a custom-designed acrylic plate as the frame for the alkali and salt compartments. Both side plates are ruthenium-coated titanium plates, and are flat. The device's internal dimensions are 3cm in length and 3cm in height, with an effective membrane area of ​​9cm². 2 The thickness of the alkali chamber, salt chamber, and polar chamber is 1.5 cm, and adjacent chambers are sealed with a 2 mm thick rubber strip. The bipolar membrane electrodialysis membrane stack is composed of bipolar membranes (BPM1 in the figure), cation exchange membranes (CEM in the figure), and bipolar membranes (BPM2 in the figure) stacked alternately, with a flow channel mesh and sealing gaskets added.

2. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as a raw material according to claim 1, characterized in that: The two-compartment electrodialysis includes an alkali compartment and a salt compartment, arranged as follows: anode-bipolar membrane-cation exchange membrane-bipolar membrane-cation exchange membrane-bipolar membrane-cathode. The cathode side of the bipolar membrane, together with the cation exchange membrane, forms the alkali compartment, and the anode side of the bipolar membrane, together with the cation exchange membrane, forms the salt compartment.

3. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as a raw material according to claim 2, characterized in that: The anode and cathode chambers are connected in parallel to the electrolyte storage tank, the alkali chamber is connected to the alkali solution tank, and the salt chamber is connected to the feed tank. The electrolyte storage tank forms a circulation loop between the anode and cathode chambers via electrolyte peristaltic pumps, the alkali solution tank and alkali chamber form a circulation loop via alkali peristaltic pumps, and the feed tank and salt chamber form a circulation loop via salt chamber peristaltic pumps. The anode and cathode chambers are connected to the positive and negative electrodes, respectively.

4. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as a raw material according to claim 3, characterized in that: The alkali chamber outlet is connected to the inside of the alkali tank via a silicone tube, and the alkali chamber inlet is connected to the outlet of the alkali chamber peristaltic pump via a silicone tube; the feed liquid outlet is connected to the inside of the feed liquid tank via a silicone tube, and the feed liquid inlet is connected to the outlet of the feed chamber peristaltic pump via a silicone tube; the electrolyte outlet is connected to the inside of the electrolyte storage tank via a silicone tube, and the electrolyte inlets are connected to the outlets of the anode and cathode chamber peristaltic pumps via silicone tubes respectively.

5. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as claimed in claim 1, comprising the following steps: A lithium citrate solution (approximately 0.5 mol / L) was introduced into the salt chamber of the bipolar electrodialysis membrane stack. Pure water was introduced into the alkali chamber. Strong electrolyte solutions (0.3 mol / L sodium sulfate solution) were introduced into the cathode and anode chambers, respectively. The feed solution in each chamber was circulated for 30 minutes using a peristaltic pump to remove air bubbles from the membrane stack. The peristaltic pump flow rate was set to 25 ml / min.

6. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as a raw material according to claim 5, characterized in that: The process operates under a constant current. A direct current is applied across the bipolar membrane electrodialysis stack. Under the influence of the direct current, hydrogen ions generated by the hydrolysis of the bipolar membrane combine with citrate ions separated from the salt chamber to form citric acid. Hydroxide ions generated by the hydrolysis of the bipolar membrane combine with lithium ions migrating from the salt chamber to the alkali chamber to form lithium hydroxide. Therefore, a lithium hydroxide solution can be obtained in the alkali chamber and a citric acid solution can be obtained in the salt chamber.

7. The bipolar membrane electrodialysis system and method for preparing citric acid and lithium hydroxide from lithium citrate as a raw material according to claim 6, characterized in that: During the treatment process of the bipolar membrane electrodialysis device, the current density is 30-80 mA / cm². 2 .

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