Vanadium metal has the characteristics of hydrogen storage, high temperature superconductivity, small fast neutron absorption cross section and corrosion resistance to liquid sodium, etc. It is used in hydrogen storage alloy and hydrogen separation film, high temperature superconducting materials, sputtering targets, coating materials and heat release elements of fuel rods in high speed breeder reactors. With the continuous expansion of the field of vanadium metal applications, the demand for vanadium metal products continues to increase, the current stage of the market has been on the vanadium sputtering target, coating vanadium tubes, 3D printing with high purity vanadium powder, vanadium wire and other vanadium products demand continues to increase 2, and these vanadium products must be processed using high purity vanadium ingot to meet the requirements, this paper describes in detail the development process of high purity vanadium ingot.

1 Process

The development technology of vanadium ingot includes the development of vanadium aluminum alloy for EB furnace melting and the preparation technology of vanadium ingot.

1.1 Development of intermediate vanadium aluminum alloy

1.1.1 Raw materials

The raw materials for the development of intermediate vanadium aluminum alloy are vanadium pentoxide, Al powder and alumina, and the corresponding components of these three raw materials are shown in Table 1-Table 3.

Table 1 Vanadium pentoxide

Table 2 Aluminum powder

Table 3 Alumina

1.1.2 Thermite reduction reaction

The vanadium pentoxide, aluminum powder and de-heating agent alumina are mixed according to the mass ratio of 1:0.5~ 0.7:0.1, after mixing evenly, dry at 120℃ for 6h, pour into the graphite crucible or copper crucible for compacting, spread the ignition agent, use alcohol to ignite the reaction, cool for 12h after the furnace. The intermediate products of vanadium and aluminum alloy are obtained and broken into certain particle size to meet the smelting requirements of EB furnace. The impurity content of vanadium and aluminum alloy is shown in Table 4.

Table 4 Impurity content of vanadium aluminum alloy

1.2 Development of metal vanadium plate

Use the EB melting furnace for melting, evenly load the vanadium and aluminum alloy intermediate products into the cleaned copper crystallizer in the EB melting furnace, close the furnace door, vacuum until the vacuum in the furnace chamber is above 2.0×10-²Pa, and start melting. The melting power is 90kW, the melting speed is 20~30 mm/min, the melting is twice, and then the cooling is 2h. Clean the furnace chamber, copper crystallizer, turn over the plate, close the furnace door, vacuum until the furnace chamber vacuum is above 2.0×10-²Pa, start melting. The melting power is 100kW, the melting speed is 20~30 mm/min, the melting is twice, and the cooling 2h after the melting is finished, the metal vanadium plate for melting vanadium ingot is obtained. Then the metal vanadium plate is cut into vanadium bars, welded into vanadium electrodes, and finally melted into metal vanadium ingot. The impurity content of vanadium plate is shown in Table 5.

Table 5 Impurity content of vanadium plate

1.3 Development of high purity metal vanadium ingot

The ingot electron beam furnace is used for melting, the metal vanadium electrode is put into the silo cleaned by the ingot electron beam furnace, the silo door is closed, and the vacuum is pumped to more than 2.0× 10-³Pa with a power of 160kW to start melting. The diameter of the electron beam running track of the two guns is controlled to about 4/5 of the diameter of the crucible. The electron beam running track of the two guns is about 1 cm away from the edge of the crucible, so that the electron beam can be automatically scanned within the running track. The vanadium dropped into the melting pool is fully melted to further remove impurities, and then a vanadium ingot with a diameter of 140 mm is cast by rotating drawing method. The second ingot is cast by the same method to form the second smelting vanadium ingot. Vanadium ingot surface smooth, no other defects. The impurity content of vanadium ingot is shown in Table 6.

Table 6 Impurity content of vanadium ingot

2 Results and discussion

2.1 Control of gas body by thermit reaction

In electron beam melting, elements 0 and N are not easy to remove, and must be strictly controlled from the production of vanadium aluminum alloy products. The solidification structure of vanadium aluminum alloy with w(V) of about 85% is composed of a large amount of V phase and a small amount of AlV3 phase, and the thermal cracking tendency is small. After the end of the reaction, the static cooling is not easy to produce cracks, and the formation of oxidation/nitriding film is reduced and reduced. If the crucible is moved or shaken at the end of the reaction before it is fully solidified, an oxidation/nitriding film will be produced inside the alloy, and a yellow, blue or black oxidation/nitriding film will be produced on the surface of the alloy ingot. In order to ensure that the alloy has a low oxygen and nitrogen content, the reaction is cooled to room temperature before removing the alloy. There are scattered oxidation/nitriding spots on the surface of the alloy, and then the surface blasting treatment, so that the production of vanadium aluminum alloy can fully meet the requirements of electron beam melting. At the same time, in the case of compaction, drying and unit reaction heat reduction, the spatter is relatively small, so the vanadium recovery rate is relatively high, and the vanadium recovery rate can reach more than 93%.

When 2.2 exabytes smelting furnace of molten melt refining rate and the number of power and determine

In high-power melting, the saturated vapor pressure of sodium, magnesium, aluminum, chromium, iron and other elements is higher than that of vanadium, which is easy to remove.

However, under high-power smelting, oxygen element is not easy to be removed. The reason is that oxygen and aluminum form a more stable Al₂O than VO, due to the stronger volatilization of Al₂O, so the deoxidation of EB melting furnace is mainly dependent on the amount of Al₂0 formed by aluminum and oxygen, due to the high temperature, aluminum promote VO thermal decomposition, and the formation of Al₂O with oxygen takes some time. In order to ensure the smooth progress of Al₂ formation reaction, it is necessary to properly slow down the evaporation rate of aluminum. When high-power smelting is used, because the temperature is too fast, aluminum is directly volatilized without time to react with oxygen, and the deoxidation reaction cannot be effectively promoted, so the phenomenon that the aluminum content is very low and the oxygen content is not reduced will occur. When using low-power melting, while the aluminum is heated and evaporated, it ensures that some aluminum reacts with oxygen to produce Al₂O, so as to promote the deoxidation reaction, so the EB melting furnace first uses the low-power, low-speed melting method of vanadium metal, and then carries out high-power melting.

2.3 cast ingot to vanadium smelting furnace of molten ingot quality of analysis

2.3.1 Influence of electron beam trajectory in electron beam furnace melting

Electron beam heating is used on the surface of the rod and molten pool. The surface temperature of the molten pool is high, and the crucible is cooled by water, so the axial temperature gradient is very large, and the radial cooling speed is also different, so it is easy to cause the uneven composition of the ingot. The radial temperature gradient of the molten pool can be controlled less by adjusting the trajectory of the electron beam in the molten pool to a reasonable position, so as to reduce the ingot composition heterogeneity. Different parts of vanadium ingot were analyzed. From the analysis results, the outer temperature of vanadium ingot is relatively low, the impurity content is relatively high, the middle temperature of electron beam heating is relatively high, the impurity content is relatively low, and the impurity content can be controlled within the required range. Therefore, the diameter of the electron beam running track of the two guns is controlled to about 4/5 of the diameter of the crucible, and the electron beam running track of the two guns is about 1cm away from the edge of the crucible, and the automatic periodic scanning method is adopted.

2.3.2 Influence of melting power and melting speed

When the melting speed is constant, the increase of the melting power is conducive to the diffusion and volatilization of impurities, but also leads to the increase of the axial temperature gradient of the molten pool and the increase of the ingot composition. Reducing the melting power is not conducive to the diffusion and volatilization of impurities, but also can reduce the axial temperature gradient of the molten pool and reduce the non-uniformity of the ingot composition. In order to eliminate such problems, relatively low melting power and low melting speed are used in a single ingot casting. This can not only prevent or reduce segregation, but also effectively remove impurities, and obtain a uniform composition of the ingot. In the secondary ingot casting, higher melting power, faster melting speed and rotary drawing method are adopted, which not only reduces segregation but also ensures the surface quality of the ingot casting.

2.3.3 Influence of impurities in melted vanadium plate

The impurities in the vanadium plate are too high, including Al, Fe, 0, N elements. If the ingot is not completely removed, the ingot will have cracks or pores. Therefore, the ingot is cast with the method of secondary melting, which can remove impurities and make the surface smooth and without defects inside. Make the ingot full for subsequent processing needs.

3 Conclusion

1) The development of high purity metal vanadium ingot for processing, the raw material must be high purity raw material, if the silicon and carbon content of vanadium pentoxide, aluminum powder, alumina is too high, enriched in vanadium aluminum alloy, it will not be easy to remove in the electron beam melting process, affecting the quality of vanadium ingot.

2) When melting vanadium plate in EB melting furnace, low power and low speed melting method must be adopted first, and then high power melting is adopted, otherwise oxygen content cannot be reduced.

3) The first smelting ingot will have the situation of uneven composition of vanadium ingot, surface cracks and internal defects. After the second melting casting, the impurity content of vanadium ingot is lower, the porosity and crack defects can be eliminated, and the surface is smooth. After ultrasonic exploration, no porosity and defects are found inside, and then it is determined to be processed.