Titanium is one of the most abundant elements in the earth’s crust, but since titanium was discovered more than 200 years ago, it has been listed as a rare metal due to difficulties in refining. 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. Since the second half of the 20th century, with the development of the aviation industry, titanium smelting has attracted people’s attention. Titanium is a high-melting point metal with a low density. The strength of titanium alloy is comparable to high-strength steel. It also has good corrosion resistance. These advantages of titanium alloy make it deservedly called “space metal”. It also has the reputation of “ocean metal”. However, so far, the global production of titanium metal is less than 1% of the production of primary aluminum, nor less than 1/10 of the production of titanium dioxide. The main reason for this phenomenon is the defect of the current titanium production method. 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. At present, titanium metal at home and abroad is mainly produced by thermal reduction of TiCl4 with magnesium, which is called Kroll method. The basic process is: a stainless steel reactor equipped with magnesium metal is heated to 950 ° C, injected with refined TiCl4, reacts and releases heat, after the reaction, the by-products MgCl2 and the remaining magnesium are vacuumed away, recycled and used for reaction After the reactor was cooled, the reactor was opened to obtain sponge titanium. The Kroll method has been criticized from the beginning for its high cost and low reduction efficiency.
The main reasons for the high production cost of this method are:
(1) Time-consuming, the whole process takes 17 days from the start of chlorination of TiO2 to the production of titanium sponge;
(2) Energy consumption, endothermic chemical reduction and exothermic chemical reduction are completed in different reactors respectively, with potential Failure to use, producing 1 ton of titanium consumes 6945kW · h;
(3) labor: labor intensity is large, the reaction needs to be carried out under vacuum and protective atmosphere, and the process is discontinuous, intermittent operation;
(4) expensive raw materials: Mg and TiCl4 accounts for about 71% of the cost;
(5) Large environmental load: TiCl4 and Cl2 are highly corrosive, the production process needs to be carried out in a closed system, and no leakage is allowed, so the equipment investment is relatively large. Half a century later, the process has not been fundamentally changed. It is still batch production, which fails to achieve continuous production, so the production cost is very high.
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 the industrialization experiment of the electrochemical method is: first, the titanium dioxide powder is formed by casting or pressure, sintered 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. 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 titanium alloys. The method has the advantages of continuous production, less investment, wide application range of products, decomposition of by-products into sodium and chlorine gas and recycling.