Preparation method of titanium scrap metal

Titanium and its alloys have excellent properties such as low density corrosion resistance and high temperature resistance. The world titanium industry is experiencing the transition from a single model with aerospace as the main market to a diversified model focusing on the development of metallurgy, energy, transportation, chemical industry, biomedicine and other civil fields. At present, the world can carry out industrialized production of titanium in only a few countries such as the United States, Japan, Russia, China and other countries, the world's total annual output of titanium is only a few tens of thousands of tons. But because of the significant strategic value of titanium and the status of the national economy, titanium will become the rise of iron, aluminum, after the "third metal", the 21st century will be the century of titanium.

Current titanium production methods current titanium production using metal thermal reduction method, which refers to the use of metal reductant (R) and metal oxides or chlorides (M X) of the reaction to prepare metal M. Already industrialized production of titanium metallurgical methods for the magnesium thermal reduction method (Kroll method) and sodium thermal reduction method (Hunter method). Because the Hunter method is more expensive than the Kroll method, the only method that is currently widely used in industry is the Kroll method, which has been criticized since its development in 1948 for its high cost and low reduction efficiency. Half a century later, the process has not fundamentally changed, is still intermittent production, failed to realize the production of continuous.

Titanium metal production method of new trends in the world titanium industry after decades of development, although the Kroll method and Hunter method for a series of improvements, but they are intermittent operation, small improvements can not significantly reduce the price of titanium. Therefore, a new, low-cost continuous process should be developed in order to fundamentally solve the problem of high production costs. To this end, researchers have conducted a large number of experiments and studies. The current research focuses on the following methods: electrochemical reduction method in order to reduce costs, people of titanium metal direct deoxidation research. Some people abroad use electrochemical methods to reduce the concentration of solid dissolved oxygen in titanium to the detection limit (500 ppm) below. They believe that in the process of electrochemical deoxidation, the deoxidizer calcium is produced in the electrolysis of calcium chloride molten salt, and O2- is precipitated in the form of CO2 or CO at the anode. This new method of high purification is used not only for deoxygenation of titanium, but also for rare earth metals such as yttrium and neodymium, and can reduce the oxygen content to 10 ppm.

Electrochemical method of the industrialization of the experimental process is: first of all, the titanium dioxide powder with casting or pressure molding, sintered for the cathode, graphite as the anode, CaCl2 as the molten salt, in graphite or titanium crucible for electrolysis. The voltage applied is 2.8V to 3.2V, which is lower than the decomposition voltage of CaCl2 (3.2V to 3.3V). After a certain time of electrolysis, the cathode changed from white to gray, and the transformation of 0.25 μm TiO2 to 12 μm titanium sponge was observed under SEM. The main reason for using calcium chloride as the molten salt is its low price and its solubility for O2-, which makes the precipitated titanium not easy to be oxidized; in addition, CaCl2 is non-toxic and non-polluting 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 quality of products is high; there will be no redox reaction between the titanium valence ions; the anode precipitation gas is pure oxygen (inert anode) or a mixture of CO and CO2 (graphite anode), which is easily controllable and non-polluting.

This method not only promotes the reduction reaction near the cathode, but also deoxidizes the titanium obtained by reduction. This method combines the direct electrolytic reduction of oxides and electrochemical deoxygenation, which is a new method of preparing titanium, and has become the most notable method in the titanium extraction process. According to the data of the paper published in the British journal Nature in 2000, it is estimated that the use of this method reduces the production cost of titanium sponge by about 13,000 U.S. dollars per ton, and the current total global production of 50,000 to 60,000 tons will save 770 million U.S. dollars per year in production costs if it is switched to the production of this electrochemical method.

Armstrong method Amstrong et al. to improve the Hunter method, making it a continuous production process. The process is as follows: TiCl4 gas is first injected into an excess of molten sodium, which acts as a cooling agent to reduce the product and carry it to the separation process. Remove the sodium and salt to get the product titanium powder. The oxygen content in the product is as low as 0.2%, reaching the standard of secondary titanium. A slight improvement of the process can produce VTi, AlTi alloys. Compared with the Hunter method, this method has the advantages of continuous production, low investment, wide range of product applications, and the by-products decomposed into sodium and chlorine can be recycled.

TiCl4 electrolytic reduction method From the perspective of electrolytic process, the use of TiCl4 electrolytic method is superior to both Kroll and Hunter methods. Therefore, from the beginning of the development of Kroll's thermal reduction method, there is the idea of transforming the titanium smelting process into an electrolytic method.

TiCl4 electrolytic reduction method is the only one that was once thought to be a possible replacement for the Kroll process, the United States, the former Soviet Union, Japan, France, Italy, China and so on have carried out long-term and in-depth research on it. The TiCl4 electrolytic reduction method is technically required to convert TiCl4 into a low-valent chloride of titanium and dissolve it in the melt, and at the same time, it is necessary to separate the cathode area from the anode area and make the electrolytic tank sealed.

Italian people have been working on TiCl4 electrolysis, they analyzed the data of chlorination electrolysis and found that when the temperature is above 900 ℃, there is no Ti2+ or Ti3+ in the electrolyte, but only Ti4+ and Ti. The electrolysis process established on this basis is as follows: TiCl4 gas is injected into a multi-layer electrolyte and absorbed. This multi-phase layer consists of ions of potassium, calcium, titanium, chlorine and fluorine as well as potassium and calcium, and separates the titanium cathode from the graphite anode. The liquid titanium generated in the lowest layer sinks to the bottom of the bath into a copper crucible with water cooling to form ingots. However, the purity of the titanium obtained by this method is not high and the efficiency is low.

Outlook has superior performance and abundant resources of titanium from the second half of the 20th century as an ideal material attention, but so far have not been from the rare metals out of the world's annual production of titanium is only tens of thousands of tons. Because the Kroll method is to reduce titanium tetrachloride with magnesium metal to get spongy titanium metal, coupled with the long process, the iteration of multiple processes and other factors, resulting in high cost of titanium sponge, affecting the application of titanium in various industries, so that it has not yet been popularized for use in many application areas. However, we believe that with the development of science and technology, titanium metal new production process development, production cost reduction, production scale expansion, the 21st century will truly become the century of titanium.