Method for preparing magnesium metal through pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride

Abstract

The invention belongs to the technical field of non-ferrous metallurgy, and particularly relates to a technological method for preparing magnesium metal through electrolysis of anhydrous magnesium chloride through pulse current and gradient temperature control optimization. The method disclosed by the invention comprises the following steps: weighing mixed molten salt (KCl-NaCl-CaCl2-MgCl2), heating and drying; the dried molten salt is added into an electrolytic cell, two-stage gradient heating is adopted, in the heating process of the second stage, when the temperature is increased to 400-500 DEG C, anhydrous MgCl2 is added, stirring is conducted while heating is conducted till the anhydrous MgCl2 is completely dissolved, and a molten salt reaction system is formed; and the cathode and the anode are immersed in a fused salt reaction system, pulse current is applied, the electrolysis time is 3-5 hours, and after electrolysis is finished, metal magnesium is obtained through aftertreatment. By adopting the method disclosed by the invention, the current efficiency is remarkably improved, the energy consumption is reduced, and the product purity is effectively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of non-ferrous metallurgy, and particularly relates to a process for preparing metal magnesium by electrolysis of anhydrous magnesium chloride through pulse current and gradient temperature control. BACKGROUND

[0002] Metal magnesium is an important light metal material. Due to its small density, high specific strength, excellent electromagnetic shielding performance, easy processing and forming, and other characteristics, it is widely used in aerospace, automobile manufacturing, electronic products, military industry, new energy and other fields. Under the development trend of global low carbonization and light weight, the demand for metal magnesium continues to grow, and the advancement and greenness of its preparation process are increasingly valued. Currently, industrial production of metal magnesium mainly relies on two technical routes of thermal reduction method and molten salt electrolysis method. Among them, the molten salt electrolysis method has become the international mainstream method for smelting magnesium due to its relatively low energy consumption, high degree of automation, high product purity and other advantages. The process uses anhydrous magnesium chloride (MgCl2) as the main raw material, and generates metal magnesium at the cathode and chlorine gas at the anode through electrolysis in a high-temperature molten state.

[0003] However, although the molten salt electrolysis method is mature, it still faces many challenges and bottlenecks in actual industrial operation, which restricts its further efficiency improvement, energy saving and green transformation. In the traditional direct current electrolysis process, the polarization phenomenon of the cathode and anode is serious, especially the concentration polarization and electrochemical polarization, which leads to an increase in the actual magnesium deposition potential, low current efficiency, and energy waste. In the current large-scale device such as a 100,000 tons / year metal magnesium project, a high current of 426 kA and a voltage of 6.08 V are used for electrolysis, and the unit power consumption for producing magnesium is still in the range of 11-13 kWh / kg, which is much higher than the theoretical power consumption value. In terms of product form control, direct current electrolysis easily produces a loose and porous magnesium deposition layer, which not only reduces the product yield, but also increases the difficulty and cost of subsequent refining. In addition, the electrolysis temperature needs to be maintained above 700℃ to maintain the flowability of the molten salt, but high temperature leads to large heat loss, and the direct current power consumption per ton of magnesium is as high as 13-15 kWh, accounting for more than 40% of the production cost.

[0004] Therefore, the current metal magnesium electrolytic preparation process urgently needs a new type of solution that is more intelligent, flexible, energy-saving and environmentally friendly, to realize the transformation from "high energy and high emission" to "high efficiency and greenness". To solve the above problems, the existing technology attempts to optimize the electrolyte composition by adding additives, or uses inert anode materials, but the effect is limited. SUMMARY

[0005] The technical problem solved by the present application is to provide a method for preparing metal magnesium by electrolysis of anhydrous magnesium chloride through pulse current coupling gradient temperature.

[0006] To solve the above technical problems, the present application provides a method for preparing metallic magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, comprising the following steps: 1) Weigh the mixed molten salt (KCl-NaCl-CaCl2-MgCl2), heat and dry (low-temperature heating and drying to remove free water), and cool to room temperature (10-25°C) to obtain the dried molten salt; The mixed molten salt is composed of the following salts with the following weight percentages: 39-41% KCl, 29-31% NaCl, 19-21% CaCl2, and 9-11% MgCl2. 2) Add the dried molten salt obtained in step 1) to an electrolytic cell and use two-stage gradient heating: the first stage is to heat from room temperature to 300±20°C and hold for 1-3 hours (preferably 2±0.2 hours); the second stage is to continue heating to the electrolysis temperature, which is 600-750°C (preferably 600-700°C); during the heating process of the second stage, when the temperature reaches 400-500°C, add anhydrous MgCl2, stir while heating (until the set electrolysis temperature is reached) until the anhydrous MgCl2 is completely dissolved, forming a molten salt reaction system; That is, continue heating to the electrolysis temperature (600-750°C) to ensure complete melting of the molten salt and reach a stable temperature; Anhydrous MgCl2: mixed molten salt = 15-25:100 by mass (preferably 20:100); 3) Immerse the cathode and anode in the molten salt reaction system and apply a pulse current, with an electrolysis time of 3-5 hours; Note: During electrolysis, a silver-white metallic luster can be observed near the cathode, which is the preliminary deposition of metallic magnesium; 4) After electrolysis, cool the reaction molten salt system obtained in step 3) (cool to ≤100°C), remove the cathode with adsorbed reaction deposits (i.e., the cathode surface has a metallic magnesium deposition layer attached), and separate the reaction deposits from the cathode. After cleaning the reaction deposits (anhydrous ethanol is used to clean the reaction deposits to remove residual molten salt), dry the reaction deposits to obtain metallic magnesium.

[0007] Note: After electrolysis, the power is first disconnected according to conventional procedures, and then cooling is performed.

[0008] After this step, weigh and test the purity of the metallic magnesium, and calculate the current efficiency based on the results. The purity of the metallic magnesium is detected by X-ray fluorescence spectroscopy (XRF).

[0009] As the improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, in the step 3), the pulse current parameters are as follows: the frequency is 0.1-1 Hz; the duty cycle is 40-60%; the peak current is 2-5 A; and the peak voltage is 4-6 V.

[0010] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, in the step 3), the immersion depth of the cathode and the anode is 35-45 mm.

[0011] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, the heating (low-temperature heating) drying of the step 1) is as follows: drying at 110-130 DEG C for 3-5 hours.

[0012] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, the temperature rising rate of the first stage and the second stage of the step 2) is 40-60 DEG C / h (preferably 50 DEG C / h).

[0013] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, The iron rod is used as the cathode, and the graphite is used as the anode.

[0014] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, the mixed molten salt is composed of the following weight percentage of salts: 40% KCl, 30% NaCl, 20% CaCl2 and 10% MgCl2.

[0015] As the further improvement of the method for preparing metal magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, in the step 3), the pulse current parameters are as follows: the frequency is set to 0.2-0.5 Hz, the duty cycle is 50%, the peak current is 3-5 A, and the peak voltage is set to 6.0 V.

[0016] The metal magnesium obtained by the method is used as the product, and the purity is greater than or equal to 98%.

[0017] In the present application, the current efficiency calculation formula is as follows: .

[0018] Among them, In order to obtain the pure metal magnesium (namely, the product quality is 100%) product, F is the Faraday constant, z is the number of electrons (magnesium is 2), M is the molar mass of magnesium, I is the average current, and t is the electrolysis time.

[0019] The application proposes a new intelligent electrolysis process of pulse current coupling gradient temperature control, which realizes efficient, low-consumption and stable target in the process of preparing magnesium metal by accurately regulating current waveform and temperature, and provides a new feasible path for the technical upgrading of molten salt electrolysis method.

[0020] The application has the following technical advantages: 1. The application controls the electrolysis process by pulse current, effectively reduces electrode polarization and side reaction loss, optimizes heat energy utilization with gradient temperature field, and can improve the current efficiency as a whole.

[0021] 2. The time gradient temperature control is designed through three stages of "low-temperature dehydration, slow phase change and stable electrolysis", and an electrolysis environment with thermodynamic balance and kinetic optimization is constructed.

[0022] It should be noted that: although high temperature (such as 600℃) is theoretically beneficial to water evaporation, the inventors fully consider that MgCl2 will start to hydrolyze significantly above 300℃. If the crystal water is not completely removed in the low-temperature stage, it will react with MgCl2 to generate MgO in the high-temperature zone, rather than escaping in the form of water vapor. Therefore, the application first sets a low-temperature heating and drying (the first stage is to heat from room temperature to 300±20℃, and keep warm for 1-3 hours).

[0023] The inventors also fully consider that: staged temperature rise is beneficial to the effective removal of water in the relatively low temperature section. If the temperature is directly raised to the electrolysis temperature, the surface water will evaporate quickly, but the internal water will be "wrapped" due to insufficient diffusion rate, which can easily cause molten salt splashing or local hydrolysis phenomenon.

[0024] 3. The application uses pulse current, which can reduce the electrode corrosion rate and effectively prolong the service life of the electrolytic cell and electrode; at the same time, the process parameters are flexible and adjustable, the system has strong adaptability, and the running stability is good.

[0025] 4. The method of the application can overcome the defects of high energy consumption, serious side reaction and low control precision of the existing molten salt electrolysis method.

[0026] In summary, the application couples pulse current regulation and gradient temperature control in the process of molten salt electrolysis of magnesium chloride, and constructs a new efficient, stable and controllable process for preparing magnesium metal. This method not only significantly improves the current efficiency and reduces the energy consumption, but also effectively improves the product purity and prolongs the service life of the equipment. The application solves the key problems of high energy consumption, many side reactions and difficult control in the traditional molten salt electrolysis process, has good laboratory repeatability and industrial application prospect, and has significant technical popularization value. DETAILED DESCRIPTION

[0027] The application will be further described below in conjunction with specific embodiments, but the protection scope of the application is not limited to this: KCl, NaCl, CaCl2, MgCl 2 、 The purity of the anhydrous MgCl2 is all ≥ 99.5% (mass %).

[0028] Room temperature refers to 10-25℃.

[0029] Example 1, a method for preparing metallic magnesium by pulse current coupling gradient temperature electrolysis of anhydrous magnesium chloride, the following steps are sequentially performed:

[0030] 1) 100g of mixed molten salt (KCl 40wt%-NaCl 30wt%-CaCl2 20wt%-MgCl2 10wt%) is placed in a drying box, and first dried at a low temperature of 120℃ for 4 hours to remove free water; Then cooled to room temperature to obtain the dried molten salt.

[0031] 2) After the dried molten salt obtained in step 1) is added to the electrolytic cell, the heating conditions of the electrolytic cell are set as follows: from room temperature to 300℃ at a rate of 50℃ / h, and maintained for 2 hours; then from 300℃ to 700℃ at a rate of 50℃ / h, wherein when the temperature is raised to 400℃, dry anhydrous MgCl2 (20g) is added while stirring, and the temperature is continuously raised to 700℃ to ensure that the molten salt is completely molten and reaches a stable temperature.

[0032] 3) Iron rod is used as the cathode, and graphite is used as the anode, the above-mentioned cathode and anode (the distance between the two is 3cm) are used in the electrolytic cell, and the electrodes are immersed in the molten salt to a depth of 40mm. The frequency of the pulse current is set to 0.5Hz, the duty cycle is 50%, the peak current is 3A, the peak voltage is set to 6.0V, and the electrolysis time is 3 hours.

[0033] 4) After electrolysis is completed, the power is turned off and the molten salt (the reaction after molten salt system obtained in step 3) is cooled to 100℃ or below. At this time, the surface of the cathode is adsorbed with the reaction product; the cathode adsorbed with the reaction product is taken out of the electrolytic cell, and the reaction product is separated from the cathode by using a conventional mechanical scraping method, and then washed with anhydrous ethanol (for the purpose of removing residual molten salt) and dried (vacuum drying at 80℃ for 2 hours), to finally obtain the product magnesium. X-ray fluorescence spectroscopy (XRF) analysis shows that the purity of the product magnesium is 99.6%, and the actual obtained metallic magnesium (100% as the same below) is 1.89g, and the current efficiency is 93.8%.

[0034] .

[0035] Example 2, the difference from example 1 is: Step 2) is changed to: After the dry molten salt obtained in step 1) is added into the electrolytic cell, the heating conditions of the electrolytic cell are set as follows: from room temperature to 300 °C at a rate of 50 °C / h, and maintaining for 2 hours; then from 300 °C to 750 °C at a rate of 50 °C / h, and maintaining for 2 hours; during the temperature rising to 400 °C, dry anhydrous MgCl2 (20 g) is added while stirring, and the temperature continues to rise to 750 °C, so as to ensure that the molten salt is completely molten and reaches a stable temperature.

[0036] The rest is the same as example 1.

[0037] The obtained results are as follows: the purity of the product magnesium is 99.3%, the actual obtained metal magnesium is 1.90 g, and the current efficiency is about 94%.

[0038] Example 3, the difference from example 1 is that: In step 3), "the peak current is 3 A" is changed to "the peak current is 5 A", and the rest is the same as example 1.

[0039] The obtained results are as follows: the purity of the product magnesium is 98.8%, the actual obtained metal magnesium is 2.85 g, and the current efficiency is about 85%.

[0040] Example 4, the difference from example 1 is that: In step 3), "the electrolysis time is 3 hours" is changed to "the electrolysis time is set to 5 hours", and the rest is the same as example 1.

[0041] The obtained results are as follows: the purity of the product magnesium is 99.1%, the actual obtained metal magnesium is 2.68 g, and the current efficiency is about 80%.

[0042] Example 5, the difference from example 1 is that: In step 3), "the frequency of the pulse current is set to 0.5 Hz" is changed to "the frequency of the pulse current is set to 0.2 Hz", and the rest is the same as example 1.

[0043] The obtained results are as follows: the purity of the product magnesium is 97.7%, the actual obtained metal magnesium is 1.75 g, and the current efficiency is about 87%.

[0044] Comparative example 1, the difference from example 1 is that: In step 2), "maintaining for 2 hours at 300 °C" is cancelled; Step 3) is changed to use conventional constant current electrolysis, using a constant current of 1.5 A and a voltage of 3 V; the electrolysis time is 3 hours; The rest is the same as example 1.

[0045] The obtained results are as follows: the purity of the metal magnesium obtained by conventional constant current electrolysis is 86.0%, the actual obtained metal magnesium is 1.24 g, and the current efficiency is about 62%.

[0046] Comparative Example 2, which differs from Example 1 in that: In Step 2, the phrase “hold at 300°C for 2 hours” is removed. The rest is identical to Example 1.

[0047] The results obtained are: the purity of the magnesium metal obtained at a single temperature is 87.0%, the actual amount of magnesium metal obtained is 1.28 g, and the current efficiency is approximately 64%.

[0048] Comparative Example 3, using only MgCl2 as the molten salt: 1) After adding 100 g of anhydrous MgCl2 to the electrolytic cell, electrolysis was performed at a temperature of 700°C, with a temperature increase rate of 50°C / h.

[0049] 2) The cathode and anode used in the electrolytic cell were the same as in Example 1, and the electrodes were immersed in the molten salt to a depth of 40 mm. Conventional constant current electrolysis was used, with a constant current of 1.5 A and a voltage of 3 V; the electrolysis time was 3 hours.

[0050] The rest is identical to Example 1.

[0051] The results obtained are: under the conditions of a pure MgCl2 molten salt, the purity of the magnesium metal is only 85.5%, and the current efficiency is 55%.

[0052] Comparative Example 4, using only MgCl2 as the molten salt: 1) After adding 100 g of anhydrous MgCl2 to the electrolytic cell, electrolysis was performed at a temperature of 700°C, with a temperature increase rate of 50°C / h.

[0053] 2) The cathode and anode used in the electrolytic cell were the same as in Example 1, and the electrodes were immersed in the molten salt to a depth of 40 mm. The frequency of the pulse current was set to 0.5 Hz, the duty cycle was 50%, the peak current was 3 A, the peak voltage was set to 6.0 V, and the electrolysis time was 3 hours.

[0054] The rest is identical to Example 1.

[0055] The results obtained are: under the conditions of a pure MgCl2 molten salt, the purity of the magnesium metal is only 85.5%, and the current efficiency is 58%.

[0056] Comparative Example 5, single addition of MgCl2 In Step 1, “100 g of a mixed molten salt (KCl 40wt% – NaCl 30wt% – CaCl2 20wt% – MgCl2 10wt%)” is changed to “120 g of a mixed molten salt (KCl 40 g, NaCl 30 g, CaCl2 20 g, MgCl2 10 g, anhydrous MgCl2 20 g”.

[0057] In Step 2, the step of "heating to 400°C and adding anhydrous MgCl2 (20 g)" is omitted. The rest is the same as Example 1.

[0058] The results obtained are: the purity of the magnesium metal obtained is 89.0% and the current efficiency is 60%.

[0059] Comparative Example 6, In Step 3), the duty cycle of the pulsed current is changed from 50% to 10% and the rest is the same as Example 1.

[0060] The results obtained are: the purity of the magnesium metal obtained is 78.0% and the current efficiency is 58%.

[0061] Finally, it should be noted that the above only lists several specific embodiments of the present application. Obviously, the present application is not limited to the above examples, but can also have many variations. All variations that can be directly derived or inferred from the disclosure of the present application by those of ordinary skill in the art should be considered within the scope of the present application.

Claims

1. A method for the production of metallic magnesium by the electrolysis of anhydrous magnesium chloride in a pulsed current coupled gradient temperature cell, characterised in that The method comprises the following steps: 1) weighing and mixing the molten salt, drying and cooling to room temperature to obtain dried molten salt; The mixed molten salt is composed of the following salts with the following weight percentages: 39-41% KCl, 29-31% NaCl, 19-21% CaCl2, and 9-11% MgCl2; 2) adding the dried molten salt obtained in step 1) into an electrolytic cell, and using two-stage gradient heating: the first stage is heating from room temperature to 300±20℃, and the second stage is continuously heating to an electrolysis temperature of 600-750℃; during the second stage, when the temperature is raised to 400-500℃, anhydrous MgCl2 is added, and the temperature is raised while stirring until the anhydrous MgCl2 is completely dissolved to form a molten salt reaction system; The mass ratio of anhydrous MgCl2 to mixed molten salt is 15-25:100; 3) immersing the cathode and anode into the molten salt reaction system, applying a pulse current, and electrolyzing for 3-5 hours; 4) after the electrolysis is completed, cooling the reaction molten salt system obtained in step 3), taking out the cathode with the reaction deposits adsorbed thereon, and cleaning the reaction deposits separated from the cathode, and drying to obtain metallic magnesium.

2. The method of claim 1, wherein the method is characterized by In step 3), the pulse current parameters are as follows: the frequency is 0.1-1 Hz, the duty cycle is 40-60%, the peak current is 2-5 A, and the peak voltage is 4-6 V.

3. The method of claim 2, wherein the method is characterized by In step 3), the immersion depth of the cathode and anode is 35-45 mm.

4. The method according to any one of claims 1-3, wherein the heating and drying in step 1) is performed at 110-130℃ for 3-5 hours.

5. The method according to claim 4, wherein the heating rate in the first stage and the second stage of step 2) is 40-60℃ / h.

6. The method according to claim 5, wherein an iron rod is used as the cathode and graphite is used as the anode.

7. The method according to claim 6, wherein the mixed molten salt is composed of the following salts with the following weight percentages: 40% KCl, 30% NaCl, 20% CaCl2, and 10% MgCl2.

8. The method according to claim 7, wherein the pulse current parameters in step 3) are as follows: the frequency is set to 0.2-0.5 Hz, the duty cycle is 50%, the peak current is 3-5 A, and the peak voltage is set to 6.0 V. ​ ​ ​ ​ ​