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2024
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02
Industrial Production Methods of Fluorine Gas.
Source:
Fluorine was first isolated by the French scientist Henri Moissan in 1886. He obtained it by electrolyzing a molten liquid of potassium fluoride and anhydrous hydrofluoric acid in a ratio of 1:12, with potassium fluoride added to overcome the non-conductivity of anhydrous hydrofluoric acid.
1. Historical Situation
Fluorine was first isolated by French scientist Henri Moisson in 1886. He obtained it by electrolyzing a molten mixture of potassium fluoride and anhydrous hydrofluoric acid in a molecular ratio of 1:12, with potassium fluoride added to overcome the non-conductivity of anhydrous hydrofluoric acid.
The Moisson electrolytic cell is a U-shaped tube made of a platinum-iridium alloy (later changed to copper), immersed in a cooling bath of chloroform to obtain cooling that prevents the volatilization of HF. Fluorine and hydrogen are produced at the anode and cathode, respectively, and the escaping fluorine gas must undergo a cooling step at -50°C for further purification. This method has a high cost due to severe corrosion, requiring 5-6g of platinum to produce 1g of fluorine, making it very expensive.
Subsequent methods for producing fluorine still used the electrolytic method invented by Moisson, but with some changes in conditions. A significant change occurred during World War I when Argo Mathers and others invented the electrolysis of the liquid obtained from molten potassium hydrogen fluoride (KF.HF). The electrolysis temperature was set at 250°C, using graphite as the anode and copper as the cathode, with the electrolytic cell made of copper. Due to the low partial pressure of HF in the gas phase above the electrolyte, it effectively suppresses the carryover of HF in F2. However, the high-temperature conditions exacerbated the corrosion of the electrolytic cell, and there were also difficulties in adding hydrofluoric acid. Before 1945, the United States and Germany used high-temperature cells with current capacities of 1kA and 2.5kA for industrial production.
In 1925, Lebeau and Damiens used molten KF.3HF (melting point 65.8°C) as the electrolyte and nickel as the anode to produce fluorine. In 1942, G.H. Cady used an electrolyte with the molecular formula KF.2HF, with nickel or carbon as the anode material to produce F2. The electrolysis temperature was controlled at 80-100°C, known as the medium temperature method.
The third type of electrolytic cell was researched by DuPont in the United States, and the Harshaw Chemical Company used KF.(8-10)HF for electrolysis at room temperature to produce fluorine. Because this type of electrolytic cell operates at the lowest temperature among the three types, it is also called the low-temperature method.
In 1942, the U.S. Office of Scientific Research and Development (O.S.R.D) organized a research group composed of major companies and research institutions to compare the advantages, disadvantages, and economics of the three types of electrolytic cells mentioned above. Researchers concluded: 1. Although low-temperature electrolytic cells were once used for industrial production of fluorine, severe corrosion of components and low current efficiency became insurmountable drawbacks. 2. High-temperature cells also showed signs of industrialization. The advantages of this type of electrolytic cell are (a) it can achieve high current density with a smaller volume for the same capacity; (b) the concentration of HF has little effect; (c) it does not require sensitive temperature control; (d) there is no significant carryover of HF; (e) low operating voltage and associated low consumption. However, due to the strong corrosion of the electrolytic cell, especially the anode components, and the difficulties in adding HF, this method was abandoned for large-scale industrial production. 3. Research on medium-temperature electrolytic cells has shown that large or small electrolytic cells operating in a KF.2HF solution at 80-90°C have satisfactory efficiency. At the same time, the corrosion of such electrolytic cells is greatly reduced, and adding HF to the electrolyte is also easily resolved. Therefore, modern fluorine production more often uses medium-temperature electrolytic cells.
Compared to nickel anodes, carbon plate anodes have higher current efficiency and almost no anode corrosion. However, other issues may arise during operation, such as variations in carbon plate quality and operating conditions, leading to production instability, which is also one of the key research areas in modern fluorine production.
2. Modern Production Methods
The modern fluorine production industry generally uses medium-temperature electrolytic cells. Due to the highly reactive chemical nature of fluorine, there are safety issues regarding the storage and transportation of products, so most factories produce other fluorides (such as sulfur hexafluoride and uranium hexafluoride) as final products. Electrolytic fluorine production is just one part of the entire production process, and the generated fluorine gas is used immediately, but this production process remains complex.
Fluorine electrolytic cells should be placed in a spacious, well-ventilated room. Anhydrous hydrofluoric acid must be supplied to the electrolytic cell, along with direct current power and cooling water, and the replenishment of the electrolyte. There should also be a separate work area near the electrolytic cell for assembly, maintenance, and repair. Different fluorine manufacturers have varying arrangements for facilities in the electrolytic cell area; some factories move the electrolytic cells for repair, while others can repair them on-site. Easily liquefiable anhydrous hydrofluoric acid can be stored in containers located far from the production workshop. The purity of the anhydrous hydrofluoric acid used in fluorine electrolytic cells must be greater than 99.95%, and moisture must be avoided from entering the electrolytic cell. In the ERDA (U.S. Energy Research and Development Administration) facility, hydrofluoric acid is vaporized in a heating container before entering the electrolytic cell, while in the ICI/BNFL (British Chemical Industry/UK Nuclear Fuel Company) facility, liquefied hydrochloric acid is directly introduced into the electrolytic cell. The electrolyte can be obtained by adding HF to a stirring tank containing high-purity KHF2. Treating the electrolyte with fluorine gas can remove impurities, especially water, reducing its content from 1000mg/g to 400mg/g. The fluorine and hydrogen gases escaping from different parts of the electrolytic cell must maintain pressure close to avoid mixing reactions under liquid seals. Generally, the H2 pressure is ±0.5kPa, and the pressure difference between fluorine and hydrogen is also maintained within ±0.5kPa. The fluorine and hydrogen gases carry 10-15% of hydrofluoric acid vapor (by volume). The carryover of the electrolyte is removed using a separator to protect valves, pumps, and other downstream equipment, and the removal of hydrofluoric acid also results in purer fluorine gas. By freezing at -85°C, the HF content in the gas from the electrolytic cell outlet can be reduced to below 2%. Due to the limitations of this technology by the high polymerization reaction of hydrofluoric acid at low temperatures, the concentration of hydrofluoric acid cannot be further reduced. Instead, subsequent absorption can be performed using a Naf tower plate absorber to reduce the concentration of hydrofluoric acid to below 0.05%. Heating to remove hydrofluoric acid can regenerate and reuse Naf. After these treatment steps, H2 can be discharged or burned off, while fluorine can be compressed for storage or directly sent to the synthesis workshop for the preparation of fluorides.
The electrolytic cell used for producing fluorine gas is currently referred to as a fluorine generator, fluorine integrated machine, or fluorine gas generation device, etc. The fluorine generator designed and processed by Luoyang Senlan Chemical Material Technology Co., Ltd. has been used by well-known research institutes, universities, and large enterprises in China, and the equipment operates effectively.
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