A method for producing metallic hafnium by molten salt electrolysis

Abstract

The application relates to a method for preparing metallic hafnium by molten salt electrolysis, and belongs to the field of hafnium metallurgy. The method is implemented by using a molten salt electrolytic cell. Under the condition of power supply operation, a hafnium compound anode is consumed by an oxidation reaction and generates hafnium ions, and a reduction reaction occurs on the surface of a cathode and generates a metallic hafnium product. The application has the advantages of short process, good adaptability, easy synthesis of electrolytic raw materials, high product purity and the like.

Description

Technical Field

[0001] This invention belongs to the field of hafnium smelting, specifically relating to a method for preparing metallic hafnium by molten salt electrolysis. Background Technology

[0002] Metallic hafnium has a high melting point (2227℃), a large thermal neutron capture cross section (115 barn), and corrosion resistance. 85% of the world's hafnium is used in metallic form as control rods and emergency shutdown rods in nuclear reactors.

[0003] Methods for preparing metallic hafnium from hafnium compounds include metallothermic reduction and molten salt electrolysis. For the metallothermic reduction method, the magnesothermic reduction of HfCl4 is commonly used industrially to produce sponge hafnium. The process is as follows: HfO2 reacts with C and Cl2 at high temperature to undergo a reductive chlorination reaction to obtain crude HfCl4. After purification, the crude HfCl4 is purified to obtain refined HfCl4. The refined HfCl4 and molten magnesium are added dropwise to a stainless steel reactor, where the reaction HfCl4(g) + 2Mg(l) = Hf(s) + 2MgCl2(l) occurs. Vacuum distillation is then used to remove impurities of MgCl2 and Mg, yielding the sponge hafnium product. This method is widely used and well-established, but it has disadvantages: the process is long, intermittent, and has low production efficiency; the magnesothermic reduction process lacks refining and impurity removal capabilities, allowing impurities from the raw materials to be carried into the sponge hafnium; and the reductive chlorination, magnesothermic reduction, and vacuum distillation operations all require high temperatures, resulting in long processing times and high energy consumption.

[0004] For molten salt electrolysis, HfCl4 is volatile and has low solubility in chloride molten salts, while HfO2 is insoluble in chlorides and has very low solubility in fluorides. Therefore, molten salt electrolysis is mostly used for the electrolytic refining of crude hafnium to remove metallic and non-metallic impurities. However, the hafnium metallurgical process of "HfO2→HfCl4→crude Hf→refined Hf" is still too long.

[0005] Drawing inspiration from the FFC-Cambridge process for preparing metallic hafnium using hafnium oxide, metallic Hf can be produced through the electro-deoxidation reaction of HfO2: HfO2 powder is pressed, sintered, and used as the cathode, with graphite as the anode and a CaCl2-based molten salt as the electrolyte. During electrolysis, HfO2 is reduced to metallic hafnium, while oxygen ions enter the molten salt and migrate to the anode to react with the graphite electrode. This method offers advantages such as a short process, low cost, omission of the chlorination process, and no corrosive gas generation. However, it lacks impurity removal capabilities; metallic impurities (e.g., Fe, Al, Zr) from the raw materials remain intact in the metallic hafnium product. Furthermore, the O2 content within the hafnium dioxide bulk material increases during the later stages of electrolysis. 2- The inability to effectively remove impurities results in a high oxygen content in the product; as electrolysis progresses, the current efficiency continues to decline, and energy consumption increases.

[0006] Based on this, there is an urgent need for a new method and device for preparing hafnium metal by molten salt electrolysis to solve the above problems. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for extracting hafnium metal by molten salt electrolysis, and the method has the advantages of low raw material requirements, simple process, strong operation adaptability, high purity of metal products, etc.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] A method for preparing hafnium metal by molten salt electrolysis, the method is implemented using an electrolytic cell, and the molten salt electrolytic cell contains a molten salt electrolyte; an anode and a cathode are respectively inserted into the molten salt electrolyte; wherein, the anode includes a hafnium compound.

[0010] The method includes: connecting the anode to the positive pole of the power supply, connecting the cathode to the negative pole of the power supply, and performing electrolysis by energization to form hafnium metal on the cathode.

[0011] Further, the hafnium compound is composed of hafnium and non-metal elements.

[0012] Preferably, the non-metal elements include one or more of oxygen, carbon, nitrogen, and sulfur.

[0013] Specifically, the hafnium compound is HfO x (1≤x≤2), HfC, HfC y O 1-y (0<y<1), HfN, HfO x N 1-x (0<x<1), HfC y N 1-y (0<y<1)HfC x O y N 1-x-y (0<x+y<1), HfS z (0.5≤z≤3), HfC y S 1-y (0<y<1) at least one of them.

[0014] More specifically, the HfO x (1≤x≤2) includes HfO, Hf3O5, Hf2O3, Hf6O 11 , Hf8O 15 , HfO2;

[0015] The HfC y O 1-y (0<y<1) includes HfC 0.5 O 0.5(i.e., Hf2CO or HfO·HfC), HfC 0.2 O 0.8 , HfC 0.6 O 0.4 ;

[0016] The aforesaid HfC y N 1-y (0 < y < 1) includes HfC 0.5 N 0.5 , HfC 0.6 N 0.4 ;

[0017] The aforesaid HfC x O y N 1-x-y (0 < x + y < 1) includes HfC 0.25 O 0.25 N 0.5 ;

[0018] The aforesaid HfS z (0.5 ≤ z ≤ 3) includes HfS, Hf2S3, HfS2, HfS3.

[0019] Preferably, when the non - metal element includes oxygen or / and carbon, the anode further includes a regulator for regulating the ratio of oxygen element to carbon element in the hafnium compound.

[0020] Further, the regulator is an oxygen - consuming agent or / and a carbon - consuming agent. The regulator is provided to avoid insufficient reaction caused by excessive oxygen (hafnium compounds rich in oxygen have poor conductivity), or excessive precipitation of carbon powder due to insufficient oxygen. Preferably, the oxygen - consuming agent includes carbon powder or / and hafnium compounds containing one or more non - metal elements among carbon, nitrogen, and sulfur; the carbon - consuming agent is a hafnium compound containing oxygen.

[0021] Preferably, the anode further includes a current collector, which is a component of the anode or added as a component of the anode. The current collector includes at least one of carbon materials, metallic hafnium, hafnium alloys, and inert metals. The combination and / or mixing of the current collector can improve the conductivity of the anode containing some hafnium compounds with poor conductivity (such as HfO2), and inhibit local overheating and fracture phenomena caused by poor conductivity. The current collector is used to fix or hold the hafnium compound.

[0022] It should be noted that carbon powder can be mixed into the anode as a current collector to enhance conductivity, or react with hafnium oxide at high temperature as an oxygen - consuming agent to form carbon monoxide or carbon dioxide gas, consuming the solid oxygen element. The purpose of controlling the addition amount of carbon powder is to improve conductivity as much as possible while avoiding the precipitation of carbon powder due to excessive carbon.

[0023] For example, for HfC 0.5 O 0.5 where the molar ratio of carbon element and oxygen element is already 1, and HfC 0.5 O 0.5 has good electrical conductivity, so there is no need to mix in carbon powder current collector / oxygen-consuming agent; for HfC 0.2 O 0.8 with certain electrical conductivity, the carbon powder current collector / oxygen-consuming agent can not be mixed in, and HfC 0.2 O 0.8 can be directly used as the anode for electrolysis reaction, or the carbon powder current collector / oxygen-consuming agent can be mixed in to promote the more complete reaction of HfC 0.2 O 0.8 , but the mixing amount does not exceed 0.6 times the molar amount of HfC 0.2 O<00,00050>; for HfO2 with poor electrical conductivity, the carbon powder current collector / oxygen-consuming agent can be mixed in, but the mixing amount does not exceed twice the molar amount of HfO2; for HfC, oxygen-consuming agents such as HfO x (1≤x≤2) or HfC y O 1-y (0<y<0.5) can be added to尽量 avoid carbon element surplus and大量 precipitation of carbon powder.

[0024] The oxidation reactions occurring in some typical hafnium-containing compound anodes are as follows:

[0025] HfO2 + C - ne - →Hf n+ + CO / CO2↑ n = 2, 3, 4

[0026] HfC + HfO2 - ne - →Hf n+ + CO / CO↑

[0027] HfC 0.5 O 0.5 - ne - →Hf n+ + CO↑

[0028] HfN - ne - →Hf n+ + N2↑

[0029] HfC 0.25 O 0.25 N 0.5 - ne<x - →Hf n+ + CO↑ + N2↑

[0030] HfS2 - ne - →Hf n+ + S2↑

[0031] The cathode is one of the following metals: hafnium, steel, tantalum, niobium, molybdenum, tungsten, platinum, etc.

[0032] The reduction reaction occurring at the cathode is as follows:

[0033] Hf n+ +ne - →Hf

[0034] Preferably, the molten salt electrolyte is composed of alkali metal halides and / or alkaline earth metal halides, and contains dissolved hafnium halide and / or hafnium halide salts.

[0035] Preferably, the alkali metal halide is one or more selected from LiCl, NaCl, KCl, LiF, NaF, KF, RbCl, CsCl, RbF, and CsF; and the alkaline earth metal halide is one or more selected from MgCl2, CaCl2, MgF2, CaF2, SrCl2, BaCl2, SrF2, and BaF2. These alkali metal halides and alkaline earth metal halides serve as supporting electrolytes and are used to dissolve hafnium-containing halides. Furthermore, both alkali metal ions and alkaline earth metal ions are more difficult to reduce than hafnium ions.

[0036] The hafnium halide is one or more selected from HfCl2, HfCl3, HfCl4, HfF2, HfF3, and HfF4; the hafnium halide is one or more selected from Na2HfCl6, K2HfCl6, Na3HfCl6, K3HfCl6, Na2HfF6, K2HfF6, Na3HfF6, and K3HfF6. These hafnium halides and / or hafnium halide are used to provide dissociated or complexed hafnium ions.

[0037] Furthermore, the hafnium ion content in the molten salt electrolyte is 1–10 wt%. Too low a hafnium ion content can easily cause side reactions of impurity ions, while too high a hafnium ion content can easily cause volatilization losses of the halides of the raw materials to be smelted in the molten salt. Therefore, the hafnium ion concentration is generally controlled between 1 and 10 wt%.

[0038] Preferably, the normal operating temperature of the electrolytic cell is 400–900°C.

[0039] Preferably, the anode current density is 0.01–1.5 A / cm². 2 Alternatively, the cathode current density can be controlled to be 0.01–2.0 A / cm². 2 .

[0040] Furthermore, the electrolysis method is unrestricted, and can be arbitrarily selected in variable voltage, constant voltage, variable current, or constant current modes.

[0041] Preferably, the electrolysis method is constant voltage, constant current, and unidirectional pulse.

[0042] The beneficial effects of this invention are as follows:

[0043] (1) Continuous production and short process. Hafnium dioxide is used as raw material to directly synthesize the hafnium compound at high temperature through carbonization, nitridation, sulfidation, and electro-deoxidation, and then metallic hafnium is obtained through molten salt electrolysis. This avoids the chlorination process of HfO2 in the traditional process. By placing multiple hafnium compound anodes in the molten salt electrolysis cell to achieve timely removal of residual anodes and timely addition of new anodes, or by continuously feeding the basket-type anode, continuous operation of molten salt electrolysis can be achieved, resulting in high production efficiency.

[0044] (2) High product purity and chlorine-free process. Impurities more inert than hafnium are difficult to dissolve during the anodic oxidation process and are removed as anode mud. Impurities more reactive than hafnium are difficult to precipitate during the cathode reduction process and are retained in the molten salt electrolyte. Under normal conditions, the purity of metallic hafnium can reach 98.5% or higher, meeting the purity requirements of nuclear-grade hafnium. No volatile or corrosive chlorine gas is generated or involved in the preparation of hafnium compounds or the anodic electrolysis process. Anode gases are easy to handle; for example, CO2 and N2 are non-toxic and harmless. CO is converted into CO2 after oxidation / combustion, and sulfur-containing compounds such as S2 can be returned to the sulfidation process of hafnium oxide raw materials.

[0045] (3) Strong operational adaptability. The electrolysis device has a simple structure, is easy to operate, and has strong adaptability. Electrolysis can operate in a wide temperature range (400~900℃), with low energy consumption at low temperatures and fast mass transfer at high temperatures. Attached Figure Description

[0046] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a schematic diagram of a molten salt electrolyzer.

[0048] Appendix Figure 1 Markings: 1-Anode; 2-Molten salt electrolyte; 3-Molten salt electrolytic cell; 4-Hafnium metal product; 5-Cathode. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0050] Figure 1 This is a schematic diagram of a molten salt electrolyzer. Please refer to [link / reference]. Figure 1 The molten salt electrolysis cell 3 contains molten salt electrolyte 2; the anode 1 and cathode 5 are respectively inserted into the molten salt electrolyte 2; wherein the anode 1 includes a hafnium compound. When the anode 1 is connected to the positive terminal of the power supply and the cathode 5 is connected to the negative terminal of the power supply, and electrolysis is performed, metallic hafnium product 4 is formed on the cathode.

[0051] Example 1

[0052] In the molten salt electrolytic cell, the anode is HfN, the cathode is a stainless steel rod, and the molten salt electrolyte is LiCl-KCl-HfCl2 (where the molar ratio of LiCl to KCl is 45:55, and the hafnium ion content is 1.1wt%).

[0053] Electrolysis was performed at a constant temperature of 480℃ under an argon atmosphere, with the initial cathode current density controlled at 0.01 A / cm². 2 After electrolysis for 24 hours, the cathode product, metallic hafnium, was extracted. Analysis showed that the purity of the hafnium was 98.7%.

[0054] Example 2

[0055] In the molten salt electrolytic cell, the anode is HfS2, the cathode is a molybdenum wire, and the molten salt electrolyte is NaCl-KCl-HfCl4 (where the molar ratio of NaCl to KCl is 1:1, and the hafnium ion content is 3.7wt%).

[0056] Electrolysis was performed at a constant temperature of 800℃ under an argon atmosphere, with the initial cathode current density controlled at 0.1 A / cm². 2 After electrolysis for 12 hours, the cathode product, metallic hafnium, was extracted. Analysis showed that the purity of the hafnium was 99.1%.

[0057] Example 3

[0058] In the molten salt electrolysis cell, the anode is HfC. 0.25 O 0.25 N 0.5 The cathode is a molybdenum wire, and the molten salt electrolyte is KF-K2HfF6 (with a hafnium ion content of 9.2 wt%).

[0059] Electrolysis was performed at a constant temperature of 900℃ under an argon atmosphere, with the initial cathode current density controlled at 0.5 A / cm². 2 After electrolysis for 10 hours, the cathode product, metallic hafnium, was extracted. Analysis showed that the purity of the hafnium was 99.8%.

[0060] Example 4

[0061] Hafnium dioxide and graphite powder were uniformly mixed in a ball mill at a molar ratio of HfO2:C of 1:2. The mixture was then pressed and sintered into blocks at 1400°C under an inert atmosphere to be used as a hafnium compound anode.

[0062] In a molten salt electrolytic cell, the aforementioned hafnium compound, bound with platinum wire, is inserted as the anode, and a tungsten wire is inserted as the cathode. The molten salt electrolyte is NaCl-CaCl2-Na3HfF6 (where the molar ratio of NaCl to CaCl2 is 1:1, and the hafnium ion content is 5.4 wt%).

[0063] Electrolysis was performed at a constant temperature of 650℃ under an inert atmosphere, with the initial cathode current density controlled at 2.0 A / cm². 2 After electrolysis for 5 hours, the cathode product, metallic hafnium, was extracted. Analysis showed that the purity of the hafnium was 98.9%.

[0064] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing metallic hafnium by molten salt electrolysis, characterized in that, The method is implemented using a molten salt electrolytic cell containing a molten salt electrolyte; the anode and cathode are respectively inserted into the molten salt electrolyte; wherein, the anode is HfC. 0.25 O 0.25 N 0.5 Or HfS2; The method includes: connecting the anode to the positive terminal of a power supply, connecting the cathode to the negative terminal of a power supply, and performing electrolysis to form metallic hafnium on the cathode.

2. The method for preparing metallic hafnium by molten salt electrolysis according to claim 1, characterized in that, The anode also includes a current collector, which is a component of the anode or added as a component of the anode.

3. The method for preparing metallic hafnium by molten salt electrolysis according to claim 1, characterized in that, The molten salt electrolyte is composed of alkali metal halides and / or alkaline earth metal halides, and contains dissolved hafnium halide and / or hafnium halide salts.

4. The method for preparing metallic hafnium by molten salt electrolysis according to claim 3, characterized in that, The alkali metal halide is one or more of LiCl, NaCl, KCl, LiF, NaF, KF, RbCl, CsCl, RbF, and CsF; the alkaline earth metal halide is one or more of MgCl2, CaCl2, MgF2, CaF2, SrCl2, BaCl2, SrF2, and BaF2; the hafnium halide is one or more of HfCl2, HfCl3, HfCl4, HfF2, HfF3, and HfF4; and the hafnium halide salt is one or more of Na2HfCl6, K2HfCl6, Na3HfCl6, K3HfCl6, Na2HfF6, K2HfF6, Na3HfF6, and K3HfF6.

5. The method for preparing metallic hafnium by molten salt electrolysis according to claim 1, characterized in that, The hafnium ion content in the molten salt electrolyte is 1–10 wt%.

6. The method for preparing metallic hafnium by molten salt electrolysis according to claim 1, characterized in that, The operating temperature inside the electrolytic cell is 400–900℃; the anode current density is 0.01–1.5 A / cm³. 2 Alternatively, the cathode current density can be controlled to be 0.01–2.0 A / cm². 2 .

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