Titanium and its alloys have excellent properties such as low density, corrosion resistance, and high temperature resistance. The world titanium industry is undergoing a single model with aerospace as the main market, and a transition to a diversified model focusing on the development of civil fields such as metallurgy, energy, transportation, chemical industry, and biomedicine. At present, there are only a few countries in the world that can carry out industrial production of titanium, such as the United States, Japan, Russia, and China. The total annual production of titanium in the world is only tens of thousands of tons. However, due to its significant strategic value and its position in the national economy, titanium will become the “third metal” that has emerged after iron and aluminum, and the 21st century will be the century of titanium.
Current titanium production method The current titanium production uses the metal thermal reduction method, which refers to the preparation of metal M using the reaction of a metal reducing agent (R) with a metal oxide or chloride (MX). The titanium metallurgical methods that have been industrially produced are the magnesium thermal reduction method (Kroll method) and the sodium thermal reduction method (Hunter method). Because the Hunter method has a higher production cost than the Kroll method, the only method currently widely used in industry is the Kroll method. The Kroll method was criticized for its high cost and low reduction efficiency since it was developed in 1948. Half a century later, the process has not changed fundamentally, it is still batch production, and it has failed to achieve continuous production.
New trends in titanium metal production methods After decades of development in the world titanium industry, although a series of improvements have been made to the Kroll method and the Hunter method, they are all intermittent operations, and small improvements cannot significantly reduce the price of titanium. Therefore, a new, low-cost continuous process should be developed to fundamentally solve the problem of high production costs. To this end, researchers conducted a lot of experiments and research. The current research focuses on the following methods: electrochemical reduction method In order to reduce costs, people have studied the direct deoxidation of titanium metal. Some people abroad use electrochemical methods to reduce the concentration of solid dissolved oxygen in titanium below the detection limit (500 ppm). They believe that in the process of electrochemical deoxygenation, the oxygen scavenger calcium is produced when the molten salt of calcium chloride is electrolyzed, and O2- is precipitated in the form of CO2 or CO at the anode. This new high-purity method is not only used for deoxidation of titanium, but also for rare earth metals such as yttrium and neodymium, and can reduce the oxygen content to 10 ppm.
The process of industrialization experiment of electrochemical method is: first, the titanium dioxide powder is formed by casting or pressure, after sintering, it is used as the cathode, graphite is used as the anode, and CaCl2 is used as the molten salt, and electrolysis is performed in the graphite or titanium crucible. The applied voltage is 2.8V ~ 3.2V, which is lower than the decomposition voltage of CaCl2 (3.2V ~ 3.3V). After a certain period of electrolysis, the cathode changed from white to gray. Observed under SEM, 0.25μm TiO2 was transformed into 12μm sponge titanium. The main reason for using calcium chloride as a molten salt is that it is low in price and has a certain solubility in O2-, so that the precipitated titanium is not easily oxidized; in addition, CaCl2 is non-toxic and has no pollution to the environment.
Compared with TiCl4 molten salt electrolysis, the raw materials used in this method are oxides rather than volatile chlorides, so the preparation process can be simplified, and the product quality is high; the oxidation-reduction reaction between titanium valence ions will not occur; anode precipitation The gas is pure oxygen (inert anode) or a mixed gas of CO and CO2 (graphite anode), which is easy to control and has no pollution.
This method not only promotes the reduction reaction near the cathode, but also deoxidizes the reduced titanium. This method combines the direct electrolytic reduction of oxides and electrochemical deoxidation. It is a new method for preparing titanium and has become the most noticeable method in the titanium extraction process. According to data from a paper published by the British Journal of Nature in 2000, using this method, the production cost of titanium sponge per ton is reduced by about 13,000 US dollars. If the current global production of 50,000 or 60,000 tons is converted to this electrochemical method, it will save 770 million annually. Production costs in US dollars.
Armstrong method Amstrong and others improved the Hunter method to make it a continuous production process. The process is as follows: First, TiCl4 gas is injected into the excess molten sodium, and the excess sodium serves to cool the reduction product and carry the product into the separation process. The product titanium powder can be obtained by removing sodium and salt. The minimum oxygen content in the product is 0.2%, which meets the standard of secondary titanium. The process is slightly improved to produce VTi and AlTi alloys. Compared with the Hunter method, this method has the advantages of continuous production, low investment, wide product application, and decomposition of by-products into sodium and chlorine.
TiCl4 electrolytic reduction method From the perspective of electrolysis process, the use of TiCl4 electrolytic method is superior to Kroll method and Hunter method. Therefore, from the beginning of Kroll’s development of the thermal reduction method, there was the idea of transforming the titanium smelting process into an electrolytic method.
TiCl4 electrolytic reduction method is the only method that was once considered to be possible to replace the Kroll process. The United States, the former Soviet Union, Japan, France, Italy, China, etc. have conducted long-term and in-depth research on it. To adopt TiCl4 electrolytic reduction method, first of all, it is necessary to dissolve TiCl4 in the low-valent chloride of titanium and dissolve it in the melt. At the same time, the cathode area and anode area must be separated and the electrolytic cell sealed.
Someone in Italy has been devoted to the research of TiCl4 electrolysis method. They analyzed the electrolysis data of chlorination method and found that when the temperature is above 900 ℃, there is no Ti2 + or Ti3 + in the electrolyte, only Ti4 + and Ti. The electrolytic process established on this basis is: TiCl4 gas is injected into the multilayer electrolyte and absorbed. This multiphase layer is composed of ions of potassium, calcium, titanium, chlorine, fluorine, potassium, calcium, etc., and separates the titanium cathode and the graphite anode. The liquid titanium generated in the lowest layer sinks to the bottom of the molten pool into a copper crucible with water cooling to form an ingot. However, the purity of titanium obtained by this method is not high, and the efficiency is low.
Prospect Titanium with superior performance and abundant resources has received attention as an ideal material since the second half of the 20th century, but so far it has not been freed from rare metals, and the annual production of titanium in the world is only tens of thousands of tons. Because the Kroll method is to reduce titanium tetrachloride by magnesium metal to obtain sponge metal titanium, coupled with the long process and multiple processes, the cost of sponge titanium remains high, which affects the application of titanium in various industries. It has not been widely used in many application fields. However, we believe that with the development of technology, the development of new production processes for titanium metal, the reduction of production costs, and the expansion of production scale, the 21st century will truly become the century of titanium.
