Tantalum can be produced by several methods, and the average recovery rate of all methods is more than 90% when the metal grade is 99.5%. However, only two methods have been successfully used in industrial production: thermal reduction of K2TaF7 with sodium and molten salt electrolysis of oxygen electrolyte.
1. Sodium reduction The reduction of K2TaF7 by sodium is based on the following exothermic reaction, which takes place in an atmosphere of argon or in vacuum at a temperature of 800 to 900℃. Different technical conditions can be used in industry.
Reduction of K2TaF7 usually occurs in contact with liquid or gaseous sodium. After reduction, or intermittently remove and grind the reduced mixture; Or the modern method is to treat the reaction mixture continuously with an inert salt (NaCl, KCl). The reaction mixture is leached by salt and repeatedly washed to produce tantalum powder. The development of reduction method is closely related to the development of high specific volume tantalum powder for capacitors. High specific capacitive tantalum powder is characterized by extremely fine particle size, large specific surface and correspondingly high purity. At present, sodium reduction has great technical advantages in producing this kind of fine tantalum powder. In addition, in normal metal products, the range of particle size variation is very small. This method can produce powder with average particle size of 1μm. The processing technology required for medium capacitive tantalum powder and all kinds of tantalum semi-finished products is the same as that required for powder treatment by molten salt electrolysis. The increasing demand for high specific capacitive tantalum powder has prompted many companies to adopt sodium reduction technology to extract metallic tantalum. At present, more than 90% of coarse tantalum powder is produced by this method.
2. Molten salt electrolysis K2TaF7 in electrolytes containing Ta2O5 molten salt electrolysis is popular mainly in the United States. Only 50-70%KCl, 20-35%KF, 5-10% K2TaF7 and 4-5%Ta2O5 electrolytes have proved successful. The electrolytic reaction takes place in an air atmosphere and in a cast iron container connected to the cathode at a temperature of 700-900 ℃. Ta2O5 and some K2TaF7, which reacts with air to absorb oxygen, are reduced, and the oxygen reacts on the graphite anode in the middle of the container to form CO2 or CO. Tantalum was strongly electrodeposited as dendritic crystals on the cathode. Ta2O5, K2TaF6 and volatile salts are added until the electrolyte is permeated by tantalum crystals. After cooling, the mixture of crystals and salt is taken out and broken, picked and washed. Then, the tantalum powder was washed with inorganic acid and graded. During this process, the average particle size of tantalum is over 20μm and the metal grade is 99.8%Ta.
