CA1202599A

CA1202599A - Upgrading titanium, zirconium and hafnium powders by plasma processing

Info

Publication number: CA1202599A
Application number: CA000420158A
Authority: CA
Inventors: Michael G. Down
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1982-06-10
Filing date: 1983-01-25
Publication date: 1986-04-01
Also published as: GB2121441B; FR2528338B1; AU1077983A; GB8303266D0; JPS58221201A; AU562396B2; GB2121441A; FR2528338A1

Abstract

ABSTRACT OF THE DISCLOSURE
A method for upgrading commercially available titanium, zirconium or hafnium powder typically containing chloride contaminant characterized by the step of contact-ing the metal powder with an inert gaseous plasma (such as the arc heated stream of an arc heater) for a sufficient time to effect physical separation of metal and contaminant salt. By operating the arc heater at temperatures in excess of the boiling point of the contaminant salt (e.g.
above 1686°K for NaC1) the purification can be assisted by vaporization of the salt. After quenching and cooling the upgraded metal powder, any residual contaminant salt on the surface of the metal can be removed by water washing.
The upgraded metal powder produced is useful in fabricating high density metal alloy parts by the blended elemental powdered metallurgical process.

Description

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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 ~ ~X~

2 50,048 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-~26~ 9

3 50,048 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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4 50,048 (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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5 50,048 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 ~2~9

6 50,048 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 lZ~26~

7 50,048 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 ~ 3~ ~

8 50,048 (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 % ~ 2 ~J~ ~

9 50,048 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, 50,048 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.

Claims

I CLAIM:

1. A process for upgrading a metal powder comprising the steps of:
(a) establishing a plasma within a plasma reactor;
(b) feeding powdered metal particles, selected from the group consisting of titanium, zirconium and hafnium, containing at least one alkali metal or alkaline earth metal halide salt contaminant, through said plasma raising the temperature of the particles above the melting point of the contaminant thus effecting a physical separation of said metal and contaminant salt;
(c) cooling said metal; and (d) recovering an upgraded metal.

2. A process of Claim 1 wherein said contaminant salt is sodium chloride.

3. A process of Claim 1 wherein said upgraded metal is quenched within said plasma reactor and thus recovered as an upgraded metal powder.

4. A process of Claim 3 wherein said physical separation is at a temperature above the boiling point of said contaminant salt, thus at least partially vaporizing said contaminant salt;

5. A process of Claim 4 wherein said physical separation results in said contaminant, at least in part, being present on the surface of said upgraded metal and wherein said surface contaminant is subsequently removed by water washing said upgraded metal.

6. In a plasma reactor wherein an inert gas stream is directed between the electrodes of an arc heater thus creating a plasma, the specific improvement compris-ing; contacting 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, within said plasma for a sufficient time to raise the tempera-ture of the powdered metal above the melting point of the contaminant and thus to effect a physical separation of metal and contaminant salt producing an upgraded metal.

7. A process of Claim 6 wherein said contaminant salt is sodium chloride at a concentration of about 0.15 weight percent.

8. A process of Claim 6 wherein said upgraded metal is quenched within said plasma reactor and thus recovered as an upgraded metal powder.

9. A process of Claim 8 wherein said physical separation is at a temperature above the boiling point of said contaminant salt, thus at least partially vaporizing said contaminant salt.

10. A process of Claim 9 wherein said physical separation results in said contaminant, at least in part, being present on the surface of said upgraded metal and wherein said surface contaminant is subsequently removed by water washing said upgraded metal.

11. A process of Claim 1, 2, 6 wherein said metal is titanium.

12. A process of Claim 1, 2, 6 wherein said metal is zirconium.

13. A process of Claim 1, 2, 6 wherein said metal is hafnium.

14. A process of Claim 2 wherein said upgraded metal is quenched within said plasma reactor and thus recovered as an upgraded metal powder.

15. A process of Claim 14 wherein said physical separation is at a temperature above the boiling point of said contaminant salt, thus at least partially vaporizing said contaminant salt.

16. A process of Claim 15 wherein said physical separation results in said contaminant, at least in part, being present on the surface of said upgraded metal and wherein said surface contaminant is subsequently removed by water washing said upgraded metal.

17. A process of Claim 7 wherein said upgraded metal is quenched within said plasma reactor and thus recovered as an upgraded metal powder.

18. A process of Claim 17 wherein said physical separation is at a temperature above the boiling point of said contaminant salt, thus at least partially vaporizing said contaminant salt.

19. A process of Claim 18 wherein said physical separation results in said contaminant, at least in part, being present on the surface of said upgraded metal and wherein said surface contaminant is subsequently removed by water washing said upgraded metal.

20. A process of Claim 7 wherein said metal is titanium.

21. A process of Claim 7 wherein said metal is zirconium.

22. A process of Claim 7 wherein said metal is hafnium.