·Niobium preparation method



·The production methods of metal niobium mainly include carbothermic reduction method to produce niobium, aluminothermic reduction method to produce niobium and sodium thermal reduction method to produce niobium powder, followed by molten salt electrolysis method to produce niobium powder and niobium halide reduction method to produce niobium.



·Vacuum carbon reduction method



·Vacuum carbon reduction is one of the main methods of producing niobium metal at home and abroad. Carbon reduction is generally carried out in a vacuum carbon tube furnace. In this method, the affinity of carbon to oxygen is greater than that of niobium to oxygen, and carbon is used as reducing agent to reduce Nb2O5 to produce niobium strip. Its advantages are high product yield (>96%), cheap reducing agent, low production cost, no by-products of sodium reduction and other methods need wet treatment, and high purity niobium bar and metal powder can be obtained.



·There are two processes in industry: direct carbon reduction and indirect carbon reduction. The niobium produced by direct carbon reduction is spongy, with large surface area, low metal impurities and nitrogen content, which is conducive to the specific capacity of niobium powder. Therefore, this process is suitable for the production of capacitor grade niobium powder. Indirect reduction is characterized by large equipment production capacity, stable process, relatively dense metal niobium bars, regular shape and size, and suitable for making niobium bars, niobium ingots and niobium processing materials.

·Aluminothermic reduction method



·The aluminothermic reduction mostly adopts the out-of-furnace method. The reduction smelting reaction is carried out in a container without external heating. After the reaction, the container is removed, the products are taken out, and the metal and slag are separated outside the furnace. The metal niobium produced by aluminothermic reduction contains more aluminum, which can be removed by vacuum heat treatment (1800 ℃), electrolytic refining and electron beam furnace melting. The niobium can reach extremely high purity after two times of electron beam furnace melting. The aluminothermic reduction method does not need to be carried out under vacuum conditions, and the equipment is simple and easy to operate, but the product purity is low, and the aluminum content in niobium is high, and the impurities need to be further purified by electron beam melting.

·Sodium thermal reduction method



·Niobium is rarely used to manufacture capacitors, so sodium reduction method is rarely used.



·Electrolytic method



·There are two methods of niobium electrolysis: molten salt electrolysis and aqueous solution electrolysis. The equipment used for producing niobium powder by molten salt electrolysis is simple and the production cost is low.



·Nitridation method



·In addition to the above methods, a nitridation method has been developed to produce niobium in recent years. This method uses niobium oxide or ferroniobium as raw material, which first reacts with ammonia or nitrogen and carbon to produce niobium nitride, and then the niobium nitride is thermally decomposed into metal niobium under vacuum and temperature of 2103~2373K.



·Niobium refining



·Niobium powder, sponge niobium or brittle niobium bars can be prepared by reducing niobium compounds, but these niobium products also contain a certain amount of impurities, including hydrogen, oxygen, nitrogen and other impurities, which are particularly easy to make niobium brittle, so it needs to be further removed. The main purposes of niobium refining are purification and densification, and these two processes are generally completed simultaneously in industrial production.



·Because niobium has a strong affinity for oxygen and carbon, containers made of oxygen-containing and carbonaceous materials cannot be used, and must be carried out under high temperature and high vacuum conditions, which not only complicates the refining process, but also requires high equipment requirements. The main methods of niobium refining include vacuum sintering, such as niobium vacuum sintering, arc smelting, such as niobium vacuum arc smelting, electron beam smelting, such as niobium electron beam smelting, etc. Among them, the purification effect of electron beam melting is the best and the most widely used. In order to obtain the best purification effect, several refining methods are often used to cooperate with each other, such as sintered niobium bars, and then electron beam smelting.