The vanadium impurities in the titanium tube are mainly VOCl3 and a small amount of VCI4. Their presence makes the TC4 titanium alloy tube appear yellow. The purpose of refining vanadium is not only to decolorize but also to remove oxygen. This is an extremely important step in the refining process.
The boiling point difference and relative volatility of titanium tubes and vanadium impurities are relatively small. For example, the boiling point difference between the two components of the titanium tube-VOCl3 series is 10°C, and the relative volatility d=1.22; while in the TC4 titanium alloy tube-VCl4 series, two components The boiling point difference is 14 degrees Celsius. Nonetheless, it is theoretically possible to use physical methods to remove vanadium impurities, such as using high-efficiency distillation columns to remove vanadium. The advantage of this method is that it does not require the use of chemical reagents, the refining process is continuous and easy to automate, and the separated VOCl3 and VCl4 can be used directly. The disadvantages are large energy consumption, large equipment investment, and the need to solve the structure of a high-power kettle, which has not yet been applied in industry.
In addition, the freezing points of the two components of the TC4 titanium alloy tube-V0Cl3 series are quite different, about 54 degrees Celsius. Therefore, the freezing crystallization method can also be used to remove VOCI3. However, freezing consumes a lot of energy, so it has not been industrially applied. For this reason, chemical methods are often used to remove vanadium. Chemical vanadium removal is to add a chemical reagent to the titanium tube to selectively reduce or precipitate the VOCl3 (or VCl4) impurity to form an insoluble alum compound that interacts with the TC4 titanium alloy tube. The separation is either selective adsorption of VoCl3 (or VCl4) to separate vanadium impurities and TC4 titanium alloy tubes from each other; or selective dissolution of VOCl2 to separate alum impurities and 7icld from each other. It can be said that a more ideal vanadium removal process with simple, long-lasting, low-cost process equipment, good working conditions, and easy implementation of continuous operations has not yet been established. This is also a topic that needs to be studied in the refining process of TC4 titanium alloy tubes.
