Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
1999-02-17
2001-03-06
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S622000
Reexamination Certificate
active
06197082
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the extractive metallurgy of refractory metals and in particular to tantalum and niobium refining. Tantalum refining can be defined as a process of impurity removal during metal consolidation by different sintering and melting techniques.
Oxygen has been considered as a major interstitial impurity in refractory metals, and the most traditional technique used to reduce its content has been deoxidization of refractory metal oxide within the metal (e.g. tantalum pentoxide) with carbon. The history of this method goes back into the beginning of the twentieth century when von Bolton obtained a patent (German Patent 216706, 1907) for producing ductile tantalum or niobium by adding carbon to the metal and heating the metal in vacuum, thereby removing carbon and oxygen as carbon monoxide. Rohn (German Patent 600369,1934) obtained a patent for production of niobium, and other refractory metals by adding a mixture of carbide and oxide to the molten metal pool while evacuating carbon monoxide.
Over the years the process for production of pure tantalum by carbon reduction of Ta
2
O
5
has been further developed and now comprises of vacuum sintering at 2000° C. followed by one or two electron-beam melts to remove not only oxygen but also other impurities such as nitrogen, carbon, iron, nickel, silicon, and virtually all other elements, except for refractory metals such as W, Mo, and Nb. Four major drawbacks of this route are:
low yield (<90% Ta, see example 1, below) due to sublimation of tantalum suboxides species;
uncontrollable yield and productivity due to inhomogeneous (not crushed) scrap feedstock;
low melting rate due to the high impurities level (basically C, N, Si, Fe, Ni, etc.); and
tantalum carbide formation due to inhomogeniety of the feedstock
Recently, Awasthi et al., (Journal of Alloys and Compounds, 1998, 265, 190-195), suggested substitution of silicon for carbon, in order to reduce tantalum losses. The experimental bench scale data demonstrated that in the temperature range of 1800-2000° C. under 10
−8
atm pressure and oxygen content <0.1% oxygen removal by SiO (g) vaporization slows down and tantalum suboxide TaO (g) starts volatilizing which may amount to significant tantalum loss and is likely to decrease a melting rate due to silicon (silica) contamination.
The tantalum industry has been facing a challenge for some time to cope with the necessity of processing and refining significant amounts of scrap, generated primarily by the capacitor industry and containing more than 40 times as much oxygen and nitrogen as normal metallurgical grade powder. This causes high tantalum losses during pyrovacuum sintering and significantly slows down the melting rate in EB furnaces and usually calls for a second EB melting (See Examples 1 and 2, below).
It is a principal object of the present invention to provide a method of extraction of tantalum and other refractory metals from such scrap or other similar high oxygen sources.
SUMMARY OF THE INVENTION
We have discovered that tantalum losses in the form of tantalum-oxygen (or other refractory metal-oxygen) bearing species can be significantly reduced, or virtually eliminated, by carrying out carbothermic reaction of refractory metal oxide (e.g., tantalum pentoxide) at a temperature slightly higher than the melting point of the metal (tantalum) while increasing the total pressure to nearly atmospheric pressure (from about 0.2 bar to 1.0 bar depending on the refractory metal(s) involved, but preferably at about 0.5 bar for tantalum from scrap capacitors as explained below). The kinetics of the carbothermic reaction can be significantly improved by crushing or otherwise particulating tantalum scraps before melting. This results in improved melt rates also.
It is a further aspect of the invention that the oxygen rich source of refractory metal is particulated and intimately and homogeneously intermixed with reducing agent particles and preferably consolidated in a coherent form and further that the mixture (whether or not consolidated) is melted incrementally—one zone at a time with a temperature gradient of heated mixture established in the solid mixture zone adjacent to the melt and reduction begins in such zone and continues as it is eventually melted.
Thermal decomposition of tantalum pentoxide can be assessed considering the following reaction and equation (1), (2):
TaO
5
→TaO (g)+TaO
2
(g)+O
2
(g) (1)
K
1
=P
TaO(g)
* P
O
2
(g)
* P
TaO
2
(g)
(2)
Where K
1
is equilibrium constant for the reaction (1). Based on Le Chatelier's Principle, the equilibrium of the reaction (1) is shifted to the left as the total pressure increases. It can also be seen from equation (2) that by reducing the value of oxygen partial pressure, one can achieve an increase of (P
Tao(g)
* P
TaO
2
(g)
) which means higher tantalum losses. This explains the fact that inert atmosphere is preferable compared to, for example, Ar-H
2
gas mixtures (see, Mimura, K., Nanjo, Materials Transactions, vol. 31 No. 4, 1990, 293-301) because the use of hydrogen will significantly reduce the oxygen partial pressure.
By increasing the operation temperature from 2000° C. to 3000° C. (where tantalum is the refractory metal involved and similar temperature selection at about melting point for other refractory metals) and milling the scrap into a uniformly blended powder one can speed up the reaction between carbon and tantalum pentoxide. Crushing of scrap before adding carbon is also an important step for this invention to:
help increase the rate of the carbothermic reaction;
allow an increase of carbon/oxygen (C/O) ratio to stoichiometric value (5 mole/mole);
result in lower residual O and C contents completely avoiding the formation of carbides (See Examples 1,2,3,4 and 5, below) and
obtain representative samples and accurate analysis of impurities of the scrap.
By finishing the process at higher temperatures one can also reduce some impurities such as nitrogen, iron, nickel. etc., which undoubtedly increase the EB melting rate.
REFERENCES:
patent: 4727928 (1988-03-01), De Vynck et al.
patent: 5972065 (1999-10-01), Dunn et al.
patent: 64-73028 (1989-03-01), None
Dorvel Robert A
Shekhter Leonid N.
Simon Ross W.
Andrews Melvyn
Cohen Jerry
H.C. Starck Inc.
Perkins Smith & Cohen
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