Composite anode for molten salt electrolysis as well as preparation method and application of composite anode

Abstract

The invention provides a composite anode for molten salt electrolysis as well as a preparation method and application of the composite anode. The composite anode comprises the following components in percentage by mass: 0.01%-10% of metal magnesium, 0.1%-6% of metal fluoride, 0-3% of metal oxide and the balance of nickel. The preparation method comprises the steps that nickel and magnesium are subjected to vacuum melting to form nickel-magnesium alloy and pulverized; carrying out ball-milling mixing on the alloy powder, metal fluoride and metal oxide; the mixed powder is subjected to hot press molding at the temperature of 600-1500 DEG C and the pressure of 50-60 MPa; processing into an anode profile; and immersing into a hydrofluoric acid solution with the concentration of 10%-45% to carry out surface passivation treatment. The composite anode is suitable for preparing fluorine gas or nitrogen trifluoride through electrolysis. Through the synergistic effect of magnesium alloying and fluoride / oxide, the corrosion resistance and structural compactness of the anode are remarkably improved, the anode effect approaches to zero, the service life is prolonged to 300 days or above, the current efficiency is high, and the technical problems that an existing carbon anode and an existing nickel anode are short in service life, rapid in dissolution and serious in pollution are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of anode materials for molten salt electrolysis, and particularly to a composite anode for molten salt electrolysis, its preparation method, and its application. Background Technology

[0002] Currently, industrial production of fluorine gas commonly uses carbon anodes, while the anodes for producing nitrogen trifluoride are generally nickel-based. Both of these electrodes have significant problems. Using carbon anodes in fluorine gas production can lead to electrolysis termination due to anodic effects, etc. Currently, the average lifespan of carbon anodes in the global electrolytic fluorine production industry is about 100 days. Electrolytic dismantling and maintenance are costly and can easily cause air pollution due to HF leakage.

[0003] With the development of technology, the semiconductor industry uses high-purity fluorine gas for cleaning or etching processes. However, during the production of fluorine gas, the combustion of carbon plates due to the anodic effect generates a large amount of carbon tetrafluoride, which is difficult to remove in existing fluorine gas purification processes. In the molten salt electrolysis process for producing nitrogen trifluoride, nickel anodes are generally used. During electrolysis, nickel dissolves too quickly, leading to severe process contamination, and the nickel anodes need to be replaced approximately every 100 days, resulting in high production costs. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a composite anode for molten salt electrolysis, its preparation method, and its application. The anode effect is close to zero, it has a long service life, and it exhibits higher current efficiency during electrolysis applications.

[0005] According to an embodiment of the present invention, in a first aspect, a composite anode for molten salt electrolysis is provided, characterized in that the anode comprises, by mass percentage: 0.01%-10% metallic magnesium, 0.1%-6% metallic fluoride, 0-3% metallic oxide, and the balance being nickel. Preferably, the mass percentage of the metallic magnesium is 0.1%-3.2%.

[0006] Preferably, the metal fluoride is nickel fluoride and / or magnesium fluoride.

[0007] Preferably, the metal oxide is nickel oxide and / or magnesium oxide.

[0008] Secondly, a method for preparing the composite anode as described above is provided, comprising the following steps: S1. Nickel and magnesium are vacuum melted to form a nickel-magnesium alloy, which is then pulverized. S2. The alloy powder is ball-milled with metal fluoride and metal oxide to obtain a mixed powder. S3. The mixed powder is hot-pressed at a temperature of 600-1500℃ and a pressure of 50-60MPa to obtain an anode blank; S4. Process the anode blank into an anode profile of the required size; S5. Immerse the anode profile in a 10%-45% hydrofluoric acid solution for 5-300 minutes to perform surface passivation treatment.

[0009] Thirdly, as mentioned above, the composite anode is used in the electrolytic production of fluorine or nitrogen trifluoride.

[0010] Compared with the prior art, the present invention has the following beneficial effects: Magnesium, as a key alloying element, has a content controlled between 0.01% and 10%, preferably 0.1% to 3.2%. Under electrolytic conditions, magnesium can be preferentially oxidized / fluorinated, forming MgF2 or MgO, which can be incorporated into or modified into the passivation film (mainly NiF2) on the nickel substrate surface. MgF2 has a high melting point and stability; its presence can refine the grains of the passivation film, improving its density and adhesion, thereby significantly enhancing the anode's resistance to fluorination corrosion. Simultaneously, the addition of an appropriate amount of magnesium may introduce lattice defects or alter the semiconductor properties of the film, helping to reduce the electron / ion transport resistance of the passivation film, thus reducing the anode overpotential. Too low a content yields no significant effect; too high a content may increase the brittleness of the matrix and potentially form excessive amounts of poorly conductive magnesium compounds, thereby increasing impedance.

[0011] Pre-add 0.1%-6% NiF2 and / or MgF2. These fluorides are uniformly distributed within the nickel-magnesium alloy matrix during hot pressing. In the early stages of electrolysis, they act as "seeds" or nucleation sites, inducing a more uniform and rapid formation of the NiF2 protective film, thus avoiding localized corrosion caused by uneven film formation in the early stages of pure nickel anodes. Furthermore, these pre-placed, stable fluoride particles can fill the matrix pores, increasing the density of the anode and hindering the penetration of corrosive media.

[0012] Add 0-3% NiO and / or MgO. The oxides may partially transform into fluorides during high-temperature hot pressing and subsequent HF acid treatment, or form composite structures with the matrix, further optimizing the microstructure and initial film properties of the anode. They also contribute to improving the material's density and the uniformity of the initial film formation. Detailed Implementation

[0013] The technical solutions of the present invention will be further described below with reference to the embodiments.

[0014] The preparation of the composite anode of the present invention includes the following key steps: Example 1-1 Formula: 2.0% metallic magnesium, 1.1% nickel fluoride, 0.2% nickel oxide. The remainder is nickel. Preparation of S1 nickel-magnesium alloy powder: Nickel blocks and magnesium ingots with a purity of 99.99% were smelted in a vacuum induction furnace in a specific ratio, with a vacuum degree better than 5×10⁻⁶. -2 The alloy ingots are melted at 1500℃ and held for 30-60 minutes to ensure uniform magnesium distribution, then cast into nickel-magnesium alloy ingots. After mechanical crushing, the alloy ingots are pulverized using a high-energy ball mill under argon protection to obtain nickel-magnesium alloy powder.

[0015] S2. Preparation of mixed powders: The above alloy powder, along with measured amounts of fluoride powder (NiF2 and / or MgF2) and oxide powder (NiO and / or MgO), is placed in a planetary ball mill at a ball-to-powder ratio of 8:1 to 12:1 and a rotation speed of 250 to 350 rpm for 4 to 8 hours. The mixture is then dried in a vacuum drying oven at 80 to 100°C to obtain a uniformly mixed composite powder.

[0016] S3, Hot pressing: The composite powder is loaded into a mold and transferred to a hot-pressing sintering furnace. Under vacuum or inert gas protection (<10 Pa), the temperature is increased to a predetermined temperature of 600-1500℃ at a rate of 10-20℃ / min, and a pressure of 50-60 MPa is applied, holding the pressure and temperature for 30-90 minutes. Subsequently, the furnace is cooled to room temperature, and the anode blank is demolded to obtain a dense anode blank.

[0017] S4. Machining: The anode blank is processed into anode profiles of the required size and shape, such as 50mm × 50mm × 5mm plate anodes or other applicable shapes, by machining methods such as wire cutting and milling.

[0018] S5. Surface passivation treatment: The anode profile is immersed in a 10%-45% hydrofluoric acid aqueous solution at room temperature for 5-300 minutes. This treatment forms a dense pre-passivation film on the anode surface, mainly composed of NiF2 and a small amount of MgF2. After removal, it is rinsed with deionized water and dried with nitrogen gas for later use. This pre-passivation treatment can significantly improve the initial electrolytic performance of the anode.

[0019] Examples 1-2 Formula: 3.2% metallic magnesium, 1.1% nickel fluoride, 0.2% nickel oxide, balance nickel.

[0020] Preparation steps: Same as in Example 1-1.

[0021] Examples 1-3 Formula: 2.0% metallic magnesium, 0.1% nickel fluoride, 3.0% nickel oxide, balance nickel.

[0022] Preparation steps: Same as in Example 1-1.

[0023] Examples 1-4 Formula: 2.0% metallic magnesium, 0.6% nickel fluoride, 0.2% nickel oxide, balance nickel.

[0024] Preparation steps: Same as in Example 1-1.

[0025] Examples 1-5 Formula: 0.1% metallic magnesium, 0.6% nickel fluoride, 0.2% nickel oxide, balance nickel.

[0026] Preparation steps: Same as in Example 1-1.

[0027] Examples 1-6 Formula: 0.5% metallic magnesium, 0.7% nickel fluoride, 0.1% magnesium fluoride, balance nickel.

[0028] Preparation steps: Same as in Example 1-1.

[0029] Examples 1-7 Formula: 1.8% metallic magnesium, 0.5% nickel oxide, 0.1% magnesium oxide, balance nickel.

[0030] Preparation steps: Same as in Example 1-1.

[0031] Examples 1-8 Formula: 2% metallic magnesium, 2.0% magnesium fluoride, 0.7% magnesium oxide, balance nickel.

[0032] Preparation steps: Same as in Example 1-1.

[0033] Examples 1-9 Formula: 1.5% metallic magnesium, 1.0% magnesium fluoride, 0.5% nickel oxide, balance nickel.

[0034] Comparative Example 1 Prepare pure nickel anodes.

[0035] Comparative Example 2 Prepare pure carbon anodes.

[0036] Example 2: Electrolysis Performance Comparison Test Under the same electrolysis conditions, using NH4F·HF molten salt as the electrolyte, at a temperature of 105℃ and an anolyte current density of 12 mA / cm², comparative tests were conducted on the anode of this invention, pure nickel anode, and commercial carbon anode. The results are shown in the table below:

[0037] As can be seen from the above test data, the composite anodes of all embodiments of the present invention exhibit excellent electrolysis performance: Service life is significantly extended: The operating time of Examples 1-1 to 1-8 is all over 300 days, with a maximum of 389 days, which is much longer than the 82 days of the pure nickel anode in Comparative Example 1 and the 45 days of the pure carbon anode in Comparative Example 2. This shows that the composite anode of the present invention has extremely high structural stability and corrosion resistance, and solves the technical bottleneck of short anode life in existing systems.

[0038] Nickel has extremely low solubility: In all embodiments, the nickel anode life exceeded 180 days, indicating that the active dissolution of the nickel matrix was effectively suppressed by magnesium alloying and the addition of fluorides / oxides, resulting in a more protective passivation film.

[0039] Fluorine gas is highly pure and stable. In the examples, the purity of nitrogen trifluoride was maintained above 99.99%, indicating that the composite anode has fewer side reactions during electrolysis, the anode effect is close to zero, and the contamination of impurities such as CF4 generated by carbon anode is avoided.

[0040] Multi-component synergistic effect: Examples 1-9 contain various fluorides and oxides. The composite anode has the longest operating time, reaching 389 days, indicating that the synergistic effect of multiple additives further optimizes the anode performance. Magnesium, as a key alloying element, preferentially forms MgF2 / MgO, enhancing the density of the passivation film; fluorides serve as nucleation sites, promoting uniform film formation; and oxides are converted into fluorides during preparation, improving the initial film quality.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A composite anode for molten salt electrolysis, characterized in that, The anode comprises, by mass percentage: 0.01%-10% of magnesium, 0.1%-6% of metal fluoride, 0-3% of metal oxide, and the balance of nickel.

2. A composite anode for molten salt electrolysis according to claim 1, characterized in that, The mass percentage of the magnesium is 0.1%-3.2%.

3. A composite anode for molten salt electrolysis according to claim 2, characterised in that, The metal fluoride is nickel fluoride and / or magnesium fluoride.

4. A composite anode for the electrolysis of molten salts according to claim 3, characterised in that, The metal oxide is nickel oxide and / or magnesium oxide.

5. A method of producing a composite anode as claimed in any one of claims 1 to 5, characterised in that, The method comprises the following steps: S1. vacuum smelting nickel with magnesium to form a nickel-magnesium alloy and powderize the alloy; S2. ball-milling the alloy powder with metal fluoride and metal oxide to obtain a mixed powder; S3. hot-pressing the mixed powder at a temperature of 600-1500 DEG C and a pressure of 50-60 MPa to obtain an anode blank; S4. machining the anode blank into an anode profile of a desired size; S5. immersing the anode profile into a hydrofluoric acid solution with a concentration of 10%-45% for 5-300 minutes to perform surface passivation treatment.

6. Use of the composite anode according to any one of claims 1-5 in electrolytic preparation of fluorine gas or nitrogen trifluoride.

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