Note: Descriptions are shown in the official language in which they were submitted.
`, ~S.~
1 50,048
UPGRADING TITANIUM, ZIRCONIUM AND HAFNIUM
POWDERS BY PLASMA PROCESSING
GOVERNMENT CONTRACT
The United States Government has rights in this
inven~ion pursuant to Contract No. F-33615 80-C-5091
awarded by the Defense Logistics Agency.
S ACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to an improved process
for upgrading titanium, zirconium and hafnium powder.
More specifically, it relates to a method o plasma puri-
fication of titanium, zirconium and hafnium powder.
Description o the Prior Art:
The properties of high corrosion resistance and
strangth combined with a relatively low density, result in
titanium alloys being ideally suited to many applications
such as the aerospace industry. Zirconium with the addi
tional property of relatively low neutron cross section
and hafnium with high neutron cross-section result in
these metals being ideally suited to many applications in
the nuclear energy field. Eowever, the widespread use of
such metals has been and continues to be severely limited
by their high cost which is a direct consequence of the
high energy consumption and the batch nature of conven-
tional metal production and of the amount of waste in
producing finished parts. For example, or every pound of
titanium fabricated in the form of a part, as much as
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seven or eight pounds of titanium can be wasted. Similarly
considerable scrap is generated in processing and fabri-
cating zirconium and hafnium, thus yenerally necessitating
a cost saving, yet expensive, step of reprocessing the
scrap.
One of the most promising techniques to circum
vent the high cost of fabricated metal parts is powder
metallurgy (PM). This technology essentially involves the
known steps of powder production ollowed by compaction
into a solid article. Historically, two differen-t pro
cesses have been developed for PM production of fabricated
metal parts. One involves hot isostatic pressing of
pre-alloyed powders and the other involves cold compaction
and subsequent sintering of blended elemental powders.
However, considerable development is still reguired to
optimize either process such that the final product pos-
sesses at least equal properties and lower cost than the
corresponding forged wrought metal part.
Since titanium powder is quite soft and ductile
and because such titanium powders are already commercially
available, the PM route to titanium alloy parts involving
the direct blending of elemental metal powders before
compaction, in principle, is very economically attractive.
Presently, however, titanium sponge from the known commer-
cial Kroll and/or Hunter processes that has been groundinto a powder exhibits a major drawback in that the high
residual impurity content, principally from chlorides
(e.g. typically 0.15 weight perrent for titanium and 0.5
weight percent for zirconium), results in high porosity in
the final PM fabricated material. For example, in a
recent article by P. R. Anderson and P. C. Eloff published
as part of The Metallurgical Society of AIME's 109th
annual mesting, February 26-28, 1980, pages 175 through
187, a high density PM (titanium, vanadium, aiuminum~
material was fabricated into a finished part by the blended
elemental processing technique and properties in excess of
the minimum specified properties of forged wrought titan-
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ium/aluminum/vanadium were achieved. However, the residualchlorine content was observed to have a strong deleterious
effect on the microstructure of the high densi~y titanium
alloy product (see conclusions, page 180). Thus, the need
for an economical method of reducing the sodium chloride
content o~ commercially available titanium powder still
exists.
It is also generally known that certain high
melting point, refractory powders can be spheroidized by
plasma processing (see for example an article by M. G.
F~y, C. B. Wolf and F. J. ~arvey, entitled "Magnetite
Spheroidization Using an Alternating Current Arc Heater",
I&EC Process Desig~ & Development, Vol. 16, pages 108+,
January, 1977, and a preceding publication by F. J. Harvey,
T. N. Meyer, R. E. Kothmann and M. G. Fey entitled "A
Model of Particle Heat Transfer in Arc Heated Gas Streams"
(published in "Proceedings of International Roundtable on
Study and Applications of Transport Phenomena in Thermal
Plasmas", IUPAC-CMRS~ Odeillo, France, 1975).
SUMMARY OE THE INVENTION
In view of the problems associated with the
presence of residual chloride content of contemporary
titanium powder and the like, I have discovered a process
for upgrading a metal powder involving the use of a plasma
heating device. Thus, the present invention involves, in
a plasma heater wherein an inert gas stream is directed
between the electrodes of the arc heater thus creating a
plasma, the specific improvement comprising; contacting a
powdered metal, sel~cted from the group consisting of Ti,
Zr, and Hf, containing at least one alkali metal or alka-
line earth metal halide salt contaminant (e.g. NaCl or
MgCl2) with the plasma for sufficient time to effect a
physical separation of metal and contaminant thus producing
an upgraded metal.
More explicitly, the process for upgrading a
metal powder according to the present invention comprises
the steps of:
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(a) establishing a plasma within a plasmareactor;
(b3 feeding a powdered metal, selected from the
group consisting of Ti, Zr, and Hf, containing at least
one alkali metal or alkaline earth metal halide salt
contaminant, through the plasma thus effecting a physical
separation of the metal and contaminant salt;
(c~ cooling the metal; and
(d) recovering the upgraded metal.
The present invention is intended to upgrade a
finely divided commercial metai. powder characterized by
containing, as contaminant, sodium chloride or the like.
Aftar passing through the plasma stream, the metal can be
quenched downstream from the plasma and thus be recovered
as an upgraded powder. Advantageously, the physical
separation (i.e., the plasma heating) is performed at a
temperature above the boiling point of the contaminant
salt (e.g. NaCl, MgC12) thus vaporizing the contaminant
salt. Consequently it should be possible to collect the
metal and salt separately. Subsequent washing can be used
if necessary to remove residual contaminant from the
surface of the purified metal.
It is a primary object of the present invention
to economically upgrade commercially available titanium
powder or sponge (or the equivalent such as Zr and Hf)
produced by the Kroll or Hunter processes such that it
will he amenable to fabrication of high density parts by
the blended elemental powdered metallurgy process. It is
a further object that the powder be upgraded in terms of
reduced chloride content. It is an a~sociated object that
the upgrading involve controlling the particle slze of the
final product including the ability, if desirable, to
produce a spheroidized purified powder. Fulfillment of
these objects and the presence and fulfillment of other
objects will be apparent upon complete reading of the
entire specification and attached claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cut-away view illustrating a
plasma reactor operating according to the present inven-
tion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention is prefer-
ably carrie~ out in a plasma reactor which incorporates a
plasma producing source. This plasma source can be an arc
heater (plasma torch) or the plasma can be generated
without the use of an electric arc; e.g by a radio fre-
quency torch. One type of plasma reactor (generally
designated by the numeral 10~ useful in the present inven-
tion is illustrated in the drawing.
As shown the plasma reactor system 10 comprises
one or more and praferably three, arc heaters 12, (two of
which are shown) which are similar in operation and con-
struction to those described in U.S. Paten~ Nos. 3,705,975
and 3,832,519. Because of the full disclosure in those
patents, the description of the arc heaters 12 is limited
herein to the basic stucture and operation. Each arc
heater 12 is a single-phase, self-stabilizing AC device
capable of power levels up to about 3500 kilowatts, or up
to about 10,000 kilowatts for the three-phase plant in-
stallation. In the practice of this invention, it is
preferred that three arc heaters 12 be provided, one for
each of the three phases of the AC power supply.
Each arc heater 12 has two annular copper el~c-
trodes 14 and 16 which are spaced at gap 18 about one
millimeter apart to accommodate a line frequency power
source of about 4 kV. An arc 20 occurs in the space or
gap 18 and incoming inert gas at inlet 22 bl~ws the arc 20
from the space into the interior o an arc chamber 24.
The gas entering at inlet 22 must be compatible with the
metal being upgraded and may be one of the gases selected
from the group consisting of an inert gas, hydrogen, or a
mixture thereof. The inert gas is preferably argon. The
arc 20 rotates at a speed of about 1000 RPS by interaction
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of the arc curre~t (up to several thousand amps AC) and a
DC magnetic field set up by internally mounted field coils
26 and 28.
The velocities yield a very high operating
efficiency for equipment of this type and the elongated
arc 20 is ultimately projected by the gas downstream into
a plenum chamber 30. Feedstock metal powders including
titanium, zirconium or hafnium are introduced under pres-
sure, through inlet 32 where they are heated by direct
contact with the plasma heated gases.
As shown in the drawing the arc heaters 12 are
mounted on a tubular member 34 and extending radially
therefrom. The member 34 is preferably cylindrical and
forms the plenum chamber 30. The member 34 is connected
to the separator 36 tangentially to enhance centrifugal
separation of thle gaseous and particulate products of the
upgrading reaction, whereby the lighter gaseous products,
such as the contaminant salts, leave the separator through
an outlet means 38, while the heavier powdered titanium,
zirconium or hafnium exit through an outlet 40 at the
lower end of the separator. The separator is cooled by
cooling jacket means 42 having a cooling water inlet 44
and an outlet 46. The resulting metal product drops into
the crucible 48 wherein an upgraded powder 50 is collected.
During operation of the arc heater, an electrical
arc is first established between the copper electrodes 14
and 16. The pressurized stream or sheath of inert gas
(such as axgon, helium or the like) is introduced through
the inlet 22 located between the electrodes. In thls
manner, the length of the plasma arc 20 is extended towards
the plenum chamber 30 and the pathway of the metal powder
to achieve the desired thermal contact and residence time.
Preferably, a minimum flow rate of inert gas is to be used
at a given particle feed rate. Particle flow rate and
plasma power (temperature) should be regulated such that
the metal melts but does not significantly vaporize as
this would lead to ultrafine material. The residence time
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and heat transfer achieved in the arc plasma can be calcu-
lated according to knoT~n prin~iples. The mathematical
modeling of the heat transfer involved in the arc heater
is more fully described in the previously mentioned F. J.
Harvey et al. IUP~C reference. Similarly, the overall
process and associated apparatus for operation of an arc
hea~er is described more fully in U.S. Patent 4,080,194
and 4,107,445.
The quenching or cooling cham~er can essentially
by any such device known in the art. Preferably, the
cooling tower walls are double walled or tube traced with
lnternal coolant circulation being employed for heat
transfer control. The overall chamber can also be inter-
nally sleeved with selected ceramic cylindrical liners to
vary the quenching conditions or to collect the product by
condensation on the wall, again as known in the art. A
variety of product collection means can be incorporated
into the plasma reactor. Thus as illustrated a cyclone
separator 36 can be used or a system of ilters, electro-
statir precipitation or the like can be employed.
It is envisioned that as the cont~minated metalpowder par~icles enter and pass through the plasma hPated
gases, the lower melting and more volatile contaminant
liquefies and tends to escape the interstices of the
sponge ines as vapors. With sufficient residence time in
the plasma stream, the metal melts and the overall process
can be viewed as a melting down and phase separation on an
individual particle basis. As ~he solid particles melt,
they contract under the influence of surfa~e tension
forces in a molten droplet. The spherical shape is re-
tained after the particle has cooled to a solid. The
manner in which the droplets are quenched and collected
will determine the exact nature of the product and several
possibilities exist. If the individual metal particles
are collected while the contaminant salt is still gaseous
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(i.e., at high temperature) with the use of a cyclone or
the like, an efficient separation and high purity product
is achieved in one step. However, such a scheme involves
the use of expensive high temperature collecting equipment
and the possibility that the resulting hot powder will
self-bond. Alternately, the upgraded metal powder exiting
the plasma stream can be rapidly quenched and collected at
a lower temperature without risk of particle agglomeration.
These alternatives are viewed as being capable of producing
either an upgraded spherical powder at temperatures above
the melting point of the metal or upgraded rough sponge
particles at lower temperatures without significantly
altering the particle size distribution. However, in this
case, the possibility of discreet contaminant particles
comingling or adhering to the metal particles or the
partial coating of the metal particles with a layer of
solid contaminant is increased. If insufficient separation
occurs, a subsequent washing step with water or mild acid
solution or the like can be employed. The aqueous acid
wash step can also be advantageously employed to remov~
any trace of iron or other contaminants simultaneously
separated by the plasma treatment of the powder.
The plasma upgrading of metal powder according
to the present invention is an extremely effective method
for the removal of halide salt contaminants, especially of
NaCl, which typically is about 0.15 weight percent for
commercial grade titanium and about 0.5 weight percent for
zirconium. However, other concentrations significantly
above these t~pical values as well as concentrations
measured in terms of a few hundred ppm or less are viewed
as being equivalent for purposes of this invention. This
process is also considered useful in reducing other unde-
sirable trace contaminants characteristic of titanium,
zirconium and hafnium production, including but not limited
to Mg, MgC12, Na, Fe and Cr containing compounds as well
as entrained H2 (if desired). Preferably to achieve
direct separation of the contaminant from the metal the
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powder temperature produced in the plasma reactor should
be above the boiling point of the contaminant but below
the boiling point or possibly even the melting point of
the base metal powder. Thus for example, the minimum
temperature for the separation of NaCl from titanium would
be about 1413C and the maximum would be about 3327C.
The actual injection of the metal powder into
the plasma reactor and into the hot gases issuing from the
arc heater (or the like) can be performed by any of the
lC conventional methods known in the art. Preferably a high
velocity is required to ensure adequate penetration of the
plasma stream. Thus a pressurized inert carrier gas such
as argon, helium or the like, as used in establishing the
desired plasma jet plume or plasma straam, can also be
advantageously used to sweep or inject the metal particles
into the plasma. Optional selected reactive gases can be
blended with the inert gas stream to achieve a desired
chemical reaction. Thus, the addition of hydrogen to the
argon stream to produce titanium hydride or the like
during the upgrading process should be considered equiva-
lent for purposes of this invention.
In the broadest sense, the method of the present
invention can be employed with any of the known plasma arc
heaters independent of the particular type of heating.
~5 Thus, the arc heater can be an alternating current or
direct current system. Similarly, various alternative
apparatuses, as known in the art, can be substituted for
the illustrated arc heater.
The advantages associated with the use of the
process according to the present invention not only include
the purification of the metal powder and the ability to
preserve the particle size distribution while controlling
the degree of spheroidization, but also the present method
is viewed as economically attractive and high].y favorable.
Routine plasma spheroidization processes typically Gonsume
only a few kilowatt hours per pound of injected material.
The theoretical minimum amount of energy to melt titanium,
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for example, is approximately 0.~ kW hr/lb. Thus, includ-
ing the pasma gas and estimated inefficiencies, the up-
grading or extra cost of processing titanium powder
according to the present invention is estimated at less
than 10 percent of the current price of sponge fines. The
small additional processing cost is more than justified by
the improved quality and the wider applicability o the
titanium powder.
Having thus described the preferred embodiments
with a certain degree of particularity, it is manifest
that many changes can be made within the details of opera-
tions, operating parameters, and implementation of the
steps without departing from the spirit and scope of this
invention. Therefore, it is to be understood that the
invention is not limited to the embodiments set forth
herein for purposes of axemplification, but is to be
limited only by the scope of the attached claims, including
the full range of equivalents to which each step thereof
is entitled.
