Note: Descriptions are shown in the official language in which they were submitted.
11D3865Z
BACKGROI~ND OF THE INVENTION
The present invention relates generally to metal
powder production and more particularly to producing high sur-
face area valve metal powders for sintering into capacitor
electrodes in an economical, high yield process.
It is well known in the art to produce tantalum
metal, for use in making capacitors, in powder forms of high
purity through reduction of a purified potassium fluotantalate
double salt tK2TaF7). This is done by adding any of various
reducing agents including liquid sodium, magnesium and calcium
to a melt of the salt to reduce the tantalum values therein.
Separation of tantalum from the reacted mass by pulverizing
and leaching to complete the metal powder production process.
The practice i9 described in U.S.Pa~ent 2,350,185 to Hellier
and Martin. Such processing, using sodium reducing agent, is
in large scale production use today for producing high purity,
high aurface area tantalum powders. After classification to
desired size cuts, the pouders are usable in manufacture of
sintered anodes for electrolytic capacitors. It is also known
in connection with such processes to dilute the tantalum con-
taining salt with alkali metal halide salt -- including sodium
chloride, potassium chloride, sodium bromi~e, potassium bromide,
sodium iodide, potassium iodide, or m~xturea thereof and the
like -- as taught in U.S. Patent 2,994,603 to Greenberg and
Fooa. Increasing the structure of tantalum powders produced
is desirable because more highly structured particles generally
yield more surface area per unit of weight or volume and
therefore more capacitance per unit of weight or volume in
the usual case.
Related criteria for the tantalum powders so produced
are high purity to avoid high leakage, good flow properties for
reliable usage in pelleting dies and automated production
-- 2 . .
io3~6sz
equipment and good pelleting properties to permit pressing
anodes at low green densities. Low sinter density after sinter-
firing produces a capacitor anode with low dissipation factor
in the finished capacitor. It is also necessary to take into
account, in evaluating tantalum powders for capacitor use, the
shrinkage characteristics of the powder. When the powders
are pressed and sintered, they may have the vulnerability that
they can only receive a very light sinter. V~ry high capaci-
tance powders, generally made by taking finer and finer cuts
of the powders, are particularly vulnerable in this regard.
When sintered at 1800C, they tend to undergo substantial
shrinkage which ~eads to loss of capacitance, and worse, un-
predictability as to electrical and mechanical properties. Yet,
the high temperature sinter is per se desired for its strengthen-
ing and cleaning benefits. Therefore it is desirable to pro-
vide a low shrinkage powder with high surface area.
To enhance the ability of the reduction run to afford
such properties, several process changes have been utilized or
proposed. Canadian Patent number 657,596 to Kelley, Rees and
2a Mettler discloses critical temperatures for the sodium reduction
of potassium fluotantalate to optimize crystalline tantalum
production, which is less sensitive to oxygen pickup than amor-
phous, spongy metal. The art has also used thoroug~ presinter-
ing of powders to increase the degree of natural agglomeration
generally present in powder produced by sodium reduction, en-
hancing powder pelleting and flow properties.
British Patent No. 694,921 of Titan Company, Inc.,
teaches the method of producing titanium metal by partially
reducing titanium tetrachloride by forming 200 gram p~llets
3a thereof, dipping such pellets into a molten alkali metal bath
to cause a reduction of the pellets to a lower chloride of
titanium and then further pelletizing the partially-reacted
103E~652
lumps into smaller masses and putting them into a reducing
agent bath to complete the reduction to free titanium metal.
Recently introduced commercial products include a
very high capacitance powder whic~ is highly structured and has
good flow properties. The highly structured products are
commercially sold under the trademarks SGV and SGVR, said pro-
ducts and trademarks being those o~ the Norton Company of
Worcester, Massachusetts, U.S.A. These products afford an
enhanced uti~ization of tantalum values in a given salt source
by producing high surface-high capacitance p~wders over a
broader size range, in contrast to the prior state-of-the-art
which involved taking finer SiSQeCUtS of reduction product
powder at lower yields.
It is an important object of the present invention
to produce tantalum and other valve metal powders in the form
of high surface area, high capacitance powders of adequate
purity, flowability, and pelletability, in high yield.
It is a further object of the invention to produce
such powders as a chemical reduction product uniformly distri-
buted through the reduction charge, consistent with thepreceding object.
It is a further object of the invention to produce
valve metal po~ders which have improved pelleting~properties,
as measured by the ability to be pressed into a binderless
compact having, in the case of tantalum, a green~density of 5.0
grams per cc. with a crush strength of at least 100 pounds for
a 1.8 gram compact and, in the case of other valve metals, a
similarly low percent of theoretical density as green density.
It is a further object of the invention to produce
a valve metal powder having a Scott Bulk Density of less than
35 grams per cubic inch (2.14 gm/cc) and distinctly advanta-
geously less than 25 grams per cubic inch (1.53 gm/cc)
lQ3~65Z
consistent with an oxygen content of no greater than 0.2
weight percent and consistent ~ith one or more of the preceding
objects.
SUMMARY OF THE INVENTION
The objects of the invention are achieved by a re-
duction process in which the valve metal -- preferably tantalum --
is produced at uniformly dispersed locations throughout the main
mass of a reduction charge and, due to processing conditions
specifically adjusted to that end, remains in such location
without substantial settling or segregation into distinct valve
metal rich and valve metal poor layers. It is believed that
the prior art processes of reduction do involve the initial
formation of highly structured metal particles, with the struc-
ture being sacrificed at later stages of the reduction as the
particles grow together in separate regions. The normal re-
duction process ~hether utilizing stirring or omitting such
stirringl is vulnerable to layering because the reaction takes
place only after formation of a molten salt pool.
In accordance with the present invention, a salt
source of the desiredvvalve metal values is divided into solid
particles of minus 10 Meqh and these particles are thoroughly
dried or otherwise treated to ~nsu-re the substantial absence
of alternative reactants to the reagents of the reduction re-
action. The dried salt charge is coated ~ith reducing agent
applied in molten form. The salt and reducing agent are inti~-
mately mixed to provide coating throughout the salt charge. The
coatings of adjacent particles are preferably in bridge con-
tacting relation to form a matrix throughout the charge mass.
The drying, coating and reduction can take place in a single
vessel or can be done in separate vessels at separate locations.
The reduction charge is heated to cause a reduction
reaction wherein the valve metal values are reduced from their
1~38652
salt source by the reducing agent. The reducing agent can be
maintain~d molten from the time of introduction to the charge
until initiation of the reaction or it can be solidified after
coating the salt particles. Although it is preferred to com-
plete addition of reducing agent to the salt charge and coating
thereof prior to initiation of the ~eduction reaction, these
steps can alternatively overlap. The reduction reaction is
5p% or more completed, ppeferably over 90% complete, before
temperature of the reduction charge is raised to reach the
lQ liquidasoQf hhe salt-charge and_preferably before reaching its
solidus temperature level. A solid, coherent but porous,
metal skeleton of bridging valve metal particles is thus formed
with salt in the pores and the salt's melting in whole or in
part does not lead to collapse of the skelton.
The salt and reducing agent are selected for their
ability to produce an exothermic reduction reaction below
the salt liquidus and preferably below its solidus.
The heating of the charge for reduction reaction
preferably includes a high temperature hold at a temperature
preferably above the metal salt mixtures freezing point (li-
quidus) and in any event above its solidus to redissolve
overly fine valve metal particles in the molten salt and re-
precipitate such valve metal values on the valve metal skeleton
as extensions of nucleating sites formed by the larger highly
structured particles therein, which are optimal for use as
electrolytic anodes or the like. The elimination in part of
fine particles also reduces vulnerability to oxygen contamina-
tion.
The reduction reaction takes place in confined con-
ditions which avoid the presence of water, oxygen,or otheralternative reactants for the reducing agent. However, the
reduction reaction charge can be diluted with inert salt to
-- 6 --
103B65Z
enhance the formation of high structure and high surface area
powder. ~ith no dilution at all, there is less surface area
of powder produced; but the ppwder turns out as a low oxygen
powder with characteristics which make it especidlly suitable
as what is known in the industry as melting grade or metal-
lurgical grade or sinter grade powder for making mill products
and fabricated parts. When dilution is used, the melting of
the diluent absorbs energy from the exothermic reaction to
stabilize temperature rise rate and the diluent also acts
as a barrier to internal pore closing within the metal mass
during the time the diluent is solid. The amount of such
dilution, if used, i5 limited only, as a practical matter, by
hhe desire for higher throughput and the danger of introducing
oxygen or other contaminants with the diluting material. The
diluting materials are selected from the class consisting of
alkali metal halide salts.
The source of valve metal values to be reduced may
be halides of the valve metal, either as single or double
salts thereof. The composItïonsinvolved are encompassed by
the formula: RXM~x+5 where R is one or more of the alkali
metals, M is valve metal, preferably tantalum or columbium,
H is one ~r more h~logen~,but including fluorine alone or in
combination with other halogens,Ax is 0 to 3 x + 5. The
reducing agents are selected from the class consisting of
sodium, potassium, lithium, magnesium, aluminum and mixtures,
alloys, compounds and amalgams thereof, such as NaK or NaBH4
or NaAlH4.
Preferably -- and with distinct advahtage -- the
charge is a mixture of K2TaF7 diluted with NaCl in a weight
ratio of NaCl to K2TaF7 up to 2.Q:l and the reducing agent is
reagent grade sodium, potassium or NaK, preferably sodium.
Preferred dilution ratios are in the ragne of 0.1:1 to 0.5:1.
-- 7 --
103865~
~ he K2TaF7 is provided in a size range of no greater
than 10 Mesh for a majority of the particles thereof and no
greater than 4 Mesh in any event, and the NaCl is provided
in a size range of no greater than 4 Mesh for the majority
thereof. Appropriate size limitations will apply to other metal
source salts and diluent salts. The particles are charged into
a ~é-~ctarvessel in the proportions above stated. They may be
charged in a repeating pattern of alternating layers thereof,
or as t~o discrete masses, or as a mixture. The reactor is
la sealed and evacuated. The charge is heated to a temperature
belo~ the spontaneous ignition temperature for the exothermic
reduction reaction and stirred 50 as to avoid unmixed pockets.
This stirring enhances heat transfer and uniformity and also
helps dry the charge. In this way ~ater vapor is removed which
would other~ise be a troublesome source of oxygen or water in
a later stage of the process. After the charge has reached
the desired temperature the vacuum is removed and the atmos-
phere is changed to a slight positive pressure such as 2
psig of inert gas such aa argon or helium. Molten sodium is
2Q added to the stirred charge in an amount corresponding to
between 95~ and lQ5% of the stoichiometric amount needed for
the reduction reaction. The continuing of stirring coats
the particles of charge ~ith molten sodium so as to essentially
disperse at least 9Q~ of the liquid sodium uniformly through
the particulate charge as coating and coating matrix. Stirring
is then stopped and heat applied with an external furnace. A't
around 300C the reaction starts and quickly permeates the
charge due to the high thermal conductance from the imbued
sodium. This exothermic reaction itself is very rapid and
3Q results in ai~ea~y adiabatic rise in temperature to between
600C and lQ00C depending on the degree of NaCl dilution.
External heating is maintained during the exothermic reaction
10386S~
to attempt to have the reactor wall temperature follow the time
temperature profile of the charge. This attenuates the degree
of solidification and remelting which might otherwise occur in
the course of temperature rise. Temperature of the charge is
stabilized in the range of 700-1100C, for ~.5 to 2 hours.
During the;~ to the melting point of the salt ~i~ut~e;(over
the range 6Q0-900C depending on dilution~ and d~ring the actual
melting process, the reduction reaction initially produces small
nucleating particles uniformly dispersed throughout the charge.
These particles grow laterally to coalesce with adjacent
nucleated particles to form a skeleton metal structure through-
out the main mass of slight coherence, ~hich is sufficient
to prevent collapse of the metal values into distinct layers,
as occurs in stireed reactor, prior art,pro-ce~sses. Durin~ the
temperature rise period, over 90% of the tantalum values in the
salt charge are reduced therefrom and collected in the porous
skeletal metal structure. During the high temperature hold
some restructuring of the skel~onooccnrs by mass transfer
through the liquid phase to remove ultrafine particles and
remaining tantalum values in the salt source are reduced there-
from and added to the skeletal metal structure which remains
porous.
After holding at the elevated temperature, unreacted
sodium is boiled off and condensed in a separate storage zone,
the reactor vessel is cooled to ambient temperature and then
opened~add the solidified reacted mass intermediate product is
chipped out and leached according to prior art procedures known
per se to produce powder or other particulate form of the reduced
metal.
The resultant tantalum powder product is characterized
by a very lo~ Scott Bult Density rS.B.D.], low oxygen, and coarse
particle size, the degrees of which are dependent on dilution
and hold temperature and time, but preferably in the range of
103~65Z
20--30 gms/in3 Cl.2-1.8 gm/cc2 S.B.D; oxygen content of 200
tp 2000 parts per million; and a distribution with 60-~90% of
the total weight of product in the size range of -40 Mesh and
+5 Microns, 1-2% of -5 Microns fine particles and the balance
of +40 Mesh particles. The -325 Mesh +5 Micron portion of the
yield ranges from 25-35%/ and has a low Scott Bulk Density which
tends to govern the Scott Bulk Density of the yield as a whole.
Various particle size cuts of the reduction product could have
Fisher Average Particle Diameters ~in microns) as follows:
Side Cut FAPD
A -40M +5 micron -- 3.7 microns
B -lOOM +5 micron -- 3.2 microns
C --325M +5 micron -- 2.4 microns
Metal contaminants derived from the vessel are low because the
present process does not require stirring a high temperature
molten mass in the reaction vessel.
Crush strengths for the A & B fractions described
above, when formed into compacts with a green density of 7.0
grams per cubic centimeter by binderless pressing, are over
500 psi and the crush strength of size cut C similarly formed
into a compact is over 300 psi. All of these size fractions
can be formed into compacts with binderless pressing having a
green density of 5.0 gm/cc ~hich is resistant to 100 psi crush-
ing pressure applied to a 1.8 gram cylindrical form compact of
0.25 inch diameter. Flowability characteristics, as determined
through Angle of Repose measurements, and specific capacitance
by weight and volume of these products approximate the flow-
ability and specific capacitance properties of the commercial
SGVR product cited above.
As used herein, the following terms have the following
meanings and/or standards.
Mesh size of particles is U.S. standard (rather than
Tyler standard2 in accordance with specification STP-447A of the
-- 10 --
~ r`
652
American Society for Testlng Material ~ASTN]. In this speci-
fication 325 Mesh corresponds to 44 microns. Scott Bulk Density
is expressed in grams per cubic inch and measured in accordance
- with ASTM B329-5BT. Angle of Repose is defined in the Dictionary
of Geological Terms (American Geological Institute, Doubleday,
196~) as corresponding to the angle between a horizontal plane
through a cone vertex and a cone side and is taken with respect
to sample tantalum powders poured through a Carney flowmeter
funnel, with an agitated tantalum wire arranged centrally in the
funnel as a stirrer, into a cup. The pouring is continued until
the powder overflows to form a cone. B.E.T. surface area
measurement in square cm. per gram is taken according to a
modification of the procedure described in the article by
Brunauer, Emmet and Teller, 60 Jl. American Chem. Soc'y 309-19
(1938) and Fisher porosity measurements and calculations of FAPD
(diameter in microns) and FPR (%) are in accordance with the
article at 12 Jl. Ind'l. Eng'g. Chem. (Anal. Ed'n) 479-482
(1940).
According to its broadest aspect, the present invention
provides a process for making powders of a metal selected from
the class consisting of tantalum and columbium from metal salt
comprising chemically reducing a particulate charge of a halide
salt, a majority of particles of which are minus 10 mesh of said
metal with a reducing agent in an exothermic reduction reaction,
utilizing a salt with a liquidus line above the initiation
temperature for said exothermic reaction and a reducing agent
selected from the class consisting of sodium, potassium, lithium,
magnesium, aluminum and mixtures, alloys, compounds and amalgams
thereof with a melting temperature below said salt liquidus,
coating said salt with molten reducing agents, heating the coated
particles to initiate said exothermic reaction and forming porous
valve metal skeleton during a period of essentially adiabatic
temperature rise of the exothermic reduction process, the
- - ~.03~6SZ ., .
skeleton having nucleating valve metal sites dispersed throughout
the charge, the skeleton being formed to a coherent state while
the salt charge temperature is below the liquidus temperature
level of the salt charge.
Numerous other objects, features and advantages and
uses of the present invention will be apparent to those skilled
in the art from the foregoing general description and from the
following specific description describing best known modes of
practicing and using the invention, and some but not all of the
variations thereof, with reference to the accompanying drawings
and table inserts in which:
Brief Description of the Drawings and Inserts
FIG. 1 is a cross-section view of a reactor containing
a full charge for the beginning of processing in accordance
with the present invention;
FIG. 2 is a cross-section view of a reactor according
to a second embodiment with separate coating and reacting
compartments;
1038~5Z
FIG. 3 is a temperature-time plot showing the tempera-
ture changes of reactor walls and reduction charge according to
a process embodiment hereof; and
FIG. 4 is a plot of specific capacitance values of
anodes formed as described in the Examples hereof, compared with
SGVR data.
Detailed Description of Preferred Embodiments of the Invention
Referring now to FIG. 1 there is sho~n a reactor vessel
10 which i5 of the general type de~crihed in the above cited
patent of Hellier et al, ~ith the exception that the side ribs
thereof are removed, the stirrer blades 40 are modified to be
of low height and afford high shear and that the power of the
driving motor is stepped up to allo~ the stirrer to work solid
particles in lieu of the molten mass stirring requireme~t of
the prior device. The vessel has a cover 14. The blades are
angled with respect to a diametral vertical plane through the
vessel, and all the blades have an aggregate height which
occupies a lower half of the vessel uhere the salt charge is
handled. After charge stirring, the blade assembly can be left
in the charge or, preferably, lifted out of the charge to occupy
the upper half of the vessel and be clear of the charge during
subsequent heating thereof and reduction reaction therein. The
vessel is in a furnace 16 with heaters 18 and an insulation
mantle 17. Sodium is fed from a supply 2a in molten form through
port 21. A reflux condenser 30, e~acuation line 28, vacuum pump
32, inert gas (e.g. argon~ source 34, and valve system 36 are
provided for gas handling at various stages of the process.
During sodium feed pressure is held at 2 psig.
At the end of a cycle, a separate portion of the
reflux condenser 30 can be used to condense boiled off unreacted
sodium into an auxiliary storage tank 35.
A pressure relief valve 36 in the form of a mercury
65Z
filledbbarometric leg, or the like, is provided for venting
argon overpressure through ~u~bling without breaking the
hermetic seal of the system during heating.
FIG. 2 shows a second em~odiment of the reactor with
separate coating and reactmon sec~ions of the vessel. An upper
hopper 101 with a motor-driven stirrer paddle 14a therein
has a salt charge 111 therein and can utilize a vacuum source
132 and inert gas source 134, via piping 128, and a sodium feed
source la9. The upper vessel has a heater la8 for heating it
to a maximum of 25QC.
Particles of the salt charge are dried and coated.
Then the coated particles are passed through valve 102 into
reaction chamber lOa which is Cinifially or subsequently) sur-
rounded by heaters 118 and insul~ting ~alls 116. The reaction
charge 112 in ves~el lQQ can be agitated if necessary for a
short time b~fore heat-up by rotating vessel 100 on a turn-table
before inserting into a furnace jacket 116/118. Thevvessel 100
is provided with radiant heat shields and at the end of a
reaction, sodium vapor c~n be pumped out by vacuum pump 132
using the same gas handling apparatus as in the FIG. 1 embodi-
ment. A reflux condenser 130 conde~ses the sodium for storage
in a separate vessel (not shounl. The central baffles 103 can
be moved aside ~or salt charge filling. Annular baffles 104
complete the cover array protecting cover 114. Vessel 101 can
be made of stainless steel and vessel laO should be made of
Inconel ~
FIG. 3 is a temperature-time plot of typical reduction
run conditions to be prescribed in accordance with the present
invention. The solid line curve indicates charge temperature and
3a the dashed line curve, reactor wall temperature. At point A the
charge is put into the reactor and stirring and heating begun to
thoroughly dry the aharge. At B-C, temperature is held at about
- 13 -
Registered Trademark
103B652
100-150C while stirring continues to dry the charge. The
stirrer is off from C to D, a hold period varia~le from zero
to any convenient time. Molten sodium is added to the charge
at D and mixed into the charge. The addition and mixing is
carried out slowly and co~tinued from point D to point E when
all the sodium has been added. Stirring is carried out from
F to G and preferably continued for at least mo minu~e~after
the last sodium addition. Reactor heating is increased after
G and approximately, at point H, the exothermic reduction
1~ reaction is initiated, and charge temperature rises ~a~idly.
Charge temperature rises rapidly and passes through the charge
solidus and liquidus temperature ranges, which for various
possible charges are indicated by S and L, respectively. Power
put into the reactor wall heaters is incEeased at the maximu~
rate to limit temperature gradient from the charge center to
the reactor ~all. The molten sodium is highly conductive and
trahsfers heat to th~ walls. The exothermically driven rise
of charga temperature carries charge temperature up to about
point X and continuing charge temperature rise is caused by
heat transfer thereto from the external~y heated reactor walls.
Reactor and charge temperatures nre stabilized between I and J
and held. Then the reactor is cooled between J and K to
ambient temperature.
~ hile the exact mechanisms of particle formation are
not entirely understood, it is believed that 90% of the tantalum
values are reduced from double salt between H and I, and probably
below S, and that between I and J, the balance of tantalum values
are reduced. From I to J, fine tantalum particles are re-
dissolved in the molten salt bath and reprecipitated as exten-
sions of the skeletal structure of tantalum in the bath made upof linked coarser, highly structured particles.
103E~65
Example 1
635 grams of Na were fused in a 6-inch diameter Inconel
reactor vessel located in a laboratory glove ~ox. The vessel
was then filled withaa dried particulate salt mixture com-
prising K2TaF7 C2167 gmi of -10 Mesh size and particulate ~aCl
(646 gm) of -10 Mesh which had been intimately premixed outside
the vessel by hand stirring. The salt mixture was added slowly
to the molten sodium with stirring. The Na/sa~t mixture tempera-
ture ~as maintained between lOQ and 150C during mixing. After
the salt particles were thoroughly coated with sodium, the
vessel was sealed and transferred to a furnace. The charge was
heated to about 30QC at which temperature the exothermic
reaction was initiated.
The temperature of the reaction products rapidly rose
to 900C, as measured by an inserted thermocouple. The reactor
walls were simultaneously being rapidly heated by the furnace,
the objective being to have the wall temperature not lag the
charge temperature or to minimize such lag. The charge temp-
erature was sta~ilized in the range of 950-1050C, and held
2~ for one hour. The reactor was cooled to room temperature in
amhient air, opened and the reaction products were crushed,
leached and classified to produce tantalum powder. The powder
yields of this and a similar experiment [lumped to provide
testing quantities] were classified and tested in various
size fractions. The corresponding porosity and Scott Bulk
Density of ~uch powders are~given in Table 1. The fractions
are identified as (1~, (2) and (3~. Powder samples of the
various fractions were pressed into 1/4 inch diameter 1.8
gram compacts of 6.0 and 7.0 gram/cc green density which had
crush strengths as indicated in Table 1.
Fractions (1~ and C21 had the capability of forming
7.0 gram/cm3 green density compacts ~ith crush strengths of 500
psi. The 6.Q gram green density compacts of the various fractions
~ -- 15 --
652
were sintered at 160QC, 1700C, 180QC, l~QQC for 8a minutes.
The resultant anodes were formed~to lOQ ~olts and wet tested in
accordance with the procedures described in Example 2 below.
- 15a -
o~ 03~652
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b o
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~ ~ W ~
~ ~ _I
O 1~
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~ ~ N
,~ a~
ol
t)
.~ ~ 1~) N
_
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dP ~ O 1` t~
~ . ~ o~
3' O
lqIn Ul
O O O
~ o o
~ ~r o ~
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-- 16 --
lQ3B652
The specific capacitances(microfarad - volts per gram~ of the
various anodes identified by their ~prticle fraction numbers ~l~
(2) and (3) are plotted in FIG. 4 for the various sinter tempera-
tures involved overlying ànline marked S~VR-4 which indicates
corresponding results for the commercial product SGVR-4 when
tested under similar condîtions. Shrinkages tvolume percentages)
of the ~intered compacts at 1600C and 190QC are given in
Table l.
Example 2
A reactor vessel, as shown in FIG. 1, was charged with
lQ8.4 lb. of K2TaF7, 48.4 lb. of NaCl and heated to the point
that the reactor walls were at 350C temperature and the~charge
temperature was about 200-300C. The charge was under vacuum
(less than 500 microns pressurel during the heating. The con-
tent~ were stirred during this heating. This charge tempera-
ture range was held for 1 hour ~ith stirring. Then the vessel
was back-filled ~ith argon to a pressure of 10 mm. Hg. above
atmospheric pressure. Then, while holding temperature and con-
tinuing stirring, 10 lh. of molten sodium were added over the
course of 5 minutes. This mixture ~as stirred for five more
minutes. 20.8 lb. of additional molten Na were then added over
10 minutes ~ith continued stirring. The stirrer was turned
off 2 minutes later. The furnace and.charge were rapidly heated
to 950C where the charge temperature was thus held for 1.0 hours.
The vessel was then cooled by ambient air cooling for a day.
The vessel was then open~d and the solid reduction products com-
prising K2TaF7, NaCl, KCl, NaF, Ta were chipped out, milled and
leached in accordance with conventional practice of the art.
A 94% yield of the theoretical weight of tantalum was obtained.
The tantalum powder produced from this metal had the
size ranges given in Table 2A below. Its -40M +5~fraction
had the Fisher porosity ratings and Scott Bulk Density indicated
in Table 2B and the chemical~Lmp ~ t~les listed in Table 2C.
The powder fractLon was pressed to 6.0 gm/cc. gyeen density
1.6 gram anodes of 1/4 inch diameter and sintered at 1650C and
formed to 100 volts in 0.1% H3~04 solution at 92C. The anodes
were wet tested in 30% ~y vol.~ ~2SO4 at 70V. for D.C. leakage
and at 0.5V.A.C. tl20 cps) for capacitance and dissipation.
The measured and calculated electrical c~aracteristics thereof
are given in Table 2B.
TABLE 2A
Powder Fraction Yield C~eight percent) for Example 2
Powder Fraction ~eight Percent
+12M .20
-12M+40M 2.8
-40M+6QM 2.7
-60M+lOaM 5.2
-laOM+325M 32.4
-325M+5~ 56.2
-5~+0~ 0.5
TABLE 2B
ao
Physical and Electrical Properties of -40M+5~Fraction (96.5w/o)
for Example 2
Powder Anodes
FAPDPR SBD L L/C C DF ESR
~(%~ (g/cc) (~a/g~a/~f~ ~f/g) (%) (ohm)
4.778.8 3916 16.4.22 74.7 11.2 1.99
TABLE 2C
Chemical Analysis of -40M+5~ Fraction for Example 2
Element: Al Ca Cr Cu Fe Mg Mn Ni Si 2 N2
ppm: <10 75 65 ~1 55 7 <1 29 <10 886 42
3~ Example 3
A reactor vessel, as shown in FIG. 1, was charged with
108.4 lb. of K2TaF7, 48.4 lb. of NaCl, mixed by stirring and
then heated to about 100C while vacuum pumping on the vessel
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652
and without stirring during the heat-up. Then the vessel was
back-filled ~ith argon to a pressure of lQ mm. Hg. above atmos-
pheric pressure. Then 10 lb. of molten sodium was added over
the course of 2 minutes ~ith the charge at about 100C. Then
the stirrer was initiated ~hile adding an additional 20.8 lb.
sodium over the course of 3 minutes. Stirring was continued for
5 more minute~ after sodium addition. Then the stirrer was
turned off. The furnace was then set for 95QC,~t~ increase
the temperature of the reaction mixture. A rapid rise in
temperature to 5~QC, indicative of an exothermic reaction,
occurred when the reaction mass ~eached 270C; as measured by
a thermocouple placed in the reaction mass and adjacent to the
reactor wall. Charge temperature was brought to 950C and held
for 1.O hours. A 92% yield of the theoretical weight of tanta-
lum uas obtained.
The tantalum powder produced from this metal had the
size ranges given in Table 3A, po~der physical properties given
in Table 3B and impurity levels given in Table 3C, and (when
pressed into anodes and processed as in Example 2~ the electri-
cal properties given in Table 3B helou.
TABLE 3A
Powder Fraction Yield (weight percent~ for Example 3
Powder Fraction~eight ~ercent
+12M .2a
-12~+40M lQ.9
-40M+60M 1~.5
-60M+lOOM 11.9
IQQM+325M 27.7
. .
-325M+5~ 37-3
-5~ 1.5
TABLE 3B
Physical and Electrical Properties of -40M+5~1Fraction t88.9w/o~
for Exam~le 3
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652
Po~derAnodes
_
FAPD FPR SBD~J L L/C C DF ESR
~k~ C%~ (g/cc~Cua/g~ (ua~fl (uf/g) (~) (ohm)
3.0 80 28.314.6 .20 73.1 11.2 2.03
TABLE 3C
Chemical Analysis for -40M+5~UFraction for Example 3
Element:Al Ca Cr Cu Fe Mg Mn Ni Si 2 N2 H2 C
ppm~10 80 11 5 18 5 < 1 25 20 3172 50 323 147
Example 4-i
A number of reaction initiation and power cut off runs
were made ~ith proce~sing as in Examp~e 1, save t~at external
heating was cutloff upon initiation of the exothermic reaction
(as indicated by a temperature rise rate of 30C. per/10 seconds,
or fasterl. In adding NaCl and K2TaF7 to the molten-sodium-
containing crucible, the NaCl ~as added first and stirred in and
K2TaF7 was added second and s~irred in. Both NaCl and K2TaF7
were prehetted to over 100C before adding.
These runs were at various dilution weight ratios of
NaCl:K2TaF7 and reached respective initiation temperatures (Ti),
maximum temperatures tTml and yielded, after leaching their
reduction products, total tantalum yields ~YT, given in weight
per cent in relation to theoretical maximum yield] and yields
of minus 40 Mesh, plus 5 micron powder ~Y40/s] as shown in
Table 4A. In each case, about 10-25 w/o of yield was in the
minus 5 micron size range.
A further series of similar runs was made without
cutting off furnace power and with elevated temperature holds
(hold temperatures TH for times t) as shown in Table 4B giving
YT and ~Y40/s] as shown.
The runs ~ith high temperature hold increased -40 mesh
+ 5 micron yield compared to the runs without holds.
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-
~03&652
TABLE 4A - EXAMPLE 4
(w/o ~
Dilution
Ratio Ti~C.~ TmtC.l YT~W/0) Y40/5(W/~)
.298 335+35gO0+10 96.0 72.9
.597 365+20825+20 93.4 66.2
113~2 31~+30 725+5 93.7 66.7
._
TABLE 4B - ExANæLE 4
tw/o~
10Dilution
Ratio TH ~Ct~minl X~ Y40/5
.298 1050 30 97.8 88.3
.597 1050 30 96.6 77.2
1.342 1050 30 97.0 73.2
.298 1050 60 97.2 88.4
.547 losa 6a 97.3 77.9
1.342 1050 60 97.4 73.1
.298 1050 240 98.3 90.0
.547 lQ50 240 98.6 78.1
201.342 1050 240 96.8 67.2
It is evident that those skilled in the art may now
make numerous uses and modifications of, and departures from
the specific embodiments described ~erein without departing
from the invention concepts. Consequently, the invention
is to be construed a~ embracing each and every novel feature
and novel combination of features present in, or possessed by
the apparatus and techniques herein disclosed and limited sole-
ly by the scope and spirit of the appended claims.
3a
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