Preparation of Ta-Al-C alloy and its subsequent conversion to pure Ta by high vacuum high temperature processing

摘要

The demand for tantalum metal has grown appreciably in recent years due to its application as high grade capacitors in electronic industry and as a material of construction in the chemical process industry. The high rectifying and dielectric properties of tantalum render it suitable for use as electrolytic capacitors and the property of the high resistance to corrosion employs it as lined vessels in chemical industries using acidic and alkaline medium. The combination of aluminothermic reduction of tantalum pentoxide and its subsequent refining by electron beam melting has made aluminothermic production of tantalum as commercially viable process. However, a host of problems also are encountered during melt refining stage. The as-reduced tantalum being very hard (hardness-350-400 VHN) needs highly energy intensive crushing step to break it down to size appropriate enough for subsequent processing. The effectiveness of electron beam melt refining of thermite tantalum is directly dependent on residual aluminium and oxygen concentration in the feed. As the aluminothermic reduction of Ta_2O_5 is carried out in presence of large amount of excess aluminium for achieving higher yield, the as-reduced thermite tantalum button ends up with high loading of aluminium. Hence, a number of melt refining steps are carried out by electron beam melting to bring down the aluminium and oxygen content in the starting tantalum feed to the acceptable limit. This further adds up to the cost of the metal and time of the production besides operational problems during melt refining stages. In the present studies, it has been possible to minimize the problems drastically encountered during crushing of thermite tantalum as well as pyro-vacuum step by incorporating a judicious amount of carbon in the thermite tantalum. A modified charge. composed of tantalum pent-oxide, reductant aluminium, heat booster potassium chlorate, lime and carbon in their appropriate proportion and intimately mixed when reacted, resulted in a product alloy of Ta-Al-C with higher tantalum recovery and low residual aluminium concentration. Carbon in the form of TaC preferentially precipitated at the grain boundaries of the thermite alloy and rendered it easily crushable. Crushed alloy product could be subjected to single or double process in the final stage of melt refining. The electron beam melted button has exhibited an average hardness value of 70 VPN and a total impurity concentration of less than 0.05 wt%.