Metal powders produced by the reduction of the oxides with...

Details Metal powders produced by the reduction of the oxides with... Metal powders produced by the reduction of the oxides with...

2001-03-16

2003-05-06

Mai, Ngoclan (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Loose particulate mixture containing metal particles

C075S352000, C075S369000, C075S245000

Reexamination Certificate

active

06558447

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
This invention relates to the production of tantalum, niobium and other metal powders and their alloys by the reduction of the corresponding metal oxide with gaseous active metals such as Mg, Ca and other elemental and compound reducing materials, in gaseous form.
Tantalum and niobium are members of a group of metals that are difficult to isolate in the free state because of the stability of their compounds, especially some of their oxides. A review of the methods developed to produce tantalum will serve to illustrate the history of a typical manufacturing process for these metals. Tantalum metal powder was first produced on a commercial scale in Germany at the beginning of the 20
th
Century by the reduction of the double salt, potassium heptafluorotantalate (K
2
TaF
7
) with sodium. Small pieces of sodium were mixed with the tantalum containing salt and sealed into a steel tube. The tube was heated at the top with a ring burner and, after ignition, the reduction proceeded quickly down the tube. The reaction mixture was allowed to cool and the solid mass, consisting of tantalum metal powder, unreacted K
2
TaF
7
and sodium, and other products of the reduction was removed by hand using a chisel. The mixture was crushed and then leached with dilute acid to separate the tantalum from the components. The process was difficult to control, dangerous, and produced a coarse, contaminated powder, but nevertheless pointed the way to what became the principal means of production of high purity tantalum in later years.
Commercial production of tantalum metal in the United States began in the 1930's. A molten mixture of K
2
TaF
7
containing tantalum oxide (Ta
2
O
5
) was electrolyzed at 700° C. in a steel retort. When the reduction was completed, the system was cooled and the solid mass removed from the electrolysis cell, and then crushed and leached to separate the coarse tantalum powder from the other reaction products. The dendritic powder was not suitable for use directly in capacitor applications.
The modern method for manufacturing tantalum was developed in the late 1950's by Hellier and Martin (Hellier, E. G. and Martin, G. L., U.S. Pat. No. 2,950,185, 1960). Following Hellier and Martin, and hundreds of subsequently described implementations or variations, a molten mixture of K
2
TaF
7
and a diluent salt, typically NaCl, is reduced with molten sodium in a stirred reactor. Using this system, control of the important reaction variables, such as reduction temperature, reaction rate, and reaction composition, was feasible. Over the years, the process was refined and perfected to the point where high quality powders with surface area exceeding 20,000 cm
2
/gm are produced and materials with surface area in the 5000-8000 cm
2
/gm range being typical. The manufacturing process still requires the removal of the solid reaction products from the retort, separation of the tantalum powder from the salts by leaching, and treatments like agglomeration to improve the physical properties. Most capacitor grade tantalum powders are also deoxidized with magnesium to minimize the oxygen content (Albrecht, W. W., Hoppe, H., Papp, V. and Wolf, R., U.S. Pat. No. 4,537,641, 1985). Artifacts of preagglomeration of primary particles to secondary particle form and doping with materials to enhance capacitance (e.g. P, N, Si, and C) are also known today.
While the reduction of K
2
TaF
7
with sodium has allowed the industry to make high performance, high quality tantalum powders thus, according to Ullmann's Encyclopedia of Industrial Chemistry, 5
th
Edition, Volume A 26, p. 80, 1993, the consumption of tantalum for capacitors had already reached a level of more than 50% of the world production of tantalum of about 1000 tons per annum, whereas there had essentially been no use of niobium for capacitors, even through the raw material base for niobium is considerably broader than that for tantalum and most of the publications on powder preparation and capacitor manufacturing methods mention niobium as well as tantalum.
Some of the difficulties of applying that process to niobium are as follows:
While the manufacturing process of the type shown in Hellier and Martin (U.S. Pat. No. 2,950,185) for the reduction of potassium heptaflorotantalate by means of sodium in a salt melt is available in principle for the production of high purity niobium powders via potassium heptafluoroniobate, it doesn't work well in practice. This is due, in part, to the difficulty of precipitating the corresponding heptafluoroniobate salts and is due, in part, to the aggressively reactive and corrosive nature of such salts, such that niobium produced by that process is very impure. Further, niobium oxide is usually unstable. See, e.g., N. F. Jackson et al, Electrocomponent Science & Technology, Vol. 1, pp. 27-37 (1974).
Accordingly, niobium has only been used in the capacitor industry to a very minor extent, predominantly in areas of with lower quality requirements.
However, niobium oxide dielectric constant is about 1,5 times as high as that of a similar tantalum oxide layer, which should allow in principle, for higher capacitance of niobium capacitors, subject to considerations of stability and other factors.
As for tantalum itself, despite the success of the K
2
TaF
7
/sodium reduction process, there are several drawbacks to this method.
It is a batch process subject to the inherent variability in the system; as a result, batch to batch consistency is difficult. Post reduction processing (mechanical and hydro-metallurgical separations, filtering) is complex, requiring considerable human and capital resources and it is time consuming. The disposal of large quantities of reaction products containing fluorides and chlorides can be a problem. Of fundamental significance, the process has evolved to a state of maturity such that the prospects for significant advances in the performance of the tantalum powder produced are limited.
Over the years, numerous attempts were made to develop alternate ways for reducing tantalum and similar metal compounds, including Nb-compounds, to the metallic state (Miller, G. L. “Tantalum and Niobium,” London, 1959, pp. 188-94; Marden, J. W. and Rich, M. H., U.S. Pat. No. 1,728,941, 1927; and Gardner, D., U.S. Pat. No. 2,516,863 1946; Hurd, U.S. Pat. No. 4,687,632). Among these were the use of active metals other than sodium, such as calcium, magnesium and aluminum and raw materials such as tantalum pentoxide and tantalum chloride. As seen in Table I, below, the negative Gibbs free energy changes indicate that the reduction of the oxides of Ta, Nb and other metals with magnesium to the metallic state is favorable; reaction rate and method determine the feasibility of using this approach to produce high quality powders on a commercial scale. To date, none of these approaches were commercialized significantly because they did not produce high quality powders. Apparently, the reason these approaches failed in the past was because the reductions were carried out by blending the reducing agents with the metal oxide. The reaction took place in contact with the molten reducing agent and under conditions of inability to control the temperature of highly exothermic reactions. Therefore, one is unable to control morphology of the products and residual reducing metal content.
TABLE 1
Gibbs Free Energy Change for
Reduction of Metal Oxides with Magnesium
M
x
O
y
(s) + yMg(g) → yMgO(s) + xM(s)
Temperature
Gibbs Free Energy Change (Kcal/mole oxide)
° C.
Ta
2
O
5
Nb
2
O
5
TiO
2
V
2
O
3
ZrO
2
WO
2
200
−219
−254
−58
−133
−22
−143
400
−215
−249
−56
−130
−21
−141
600
−210
−244
−55
−126
−20
−139
800
−202
−237
−52
−122
−18
−137
1000 
−195
−229
−50
−116
−15
−134
1200 
−186
−221
−47
−111
−13
−131
1400 
−178
−212
−45

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