Process for producing fine spherical particles having a low oxygen content

A process is disclosed for producing fine spherical particles from a starting fine powder material which comprises entraining the powder material in a carrier gas, passing the powder material and the carrier gas through a high temperature zone, and maintaining the powder in the high temperature zone for a sufficient time to melt at least about 50% by weight of the particles of the powder and to form spherical particles of the melted portion, allowing the high temperature treated material to come in contact with a reducing atmosphere created by a stream of hydrogen gas, and thereafter rapidly solidifying the resulting high temperature treated material to form spherical particles wherein the oxygen content of the spherical particles is reduced by greater than about 10% by weight from the starting powder.

This invention relates to a process for producing fine spherical particles 
having a low oxygen content wherein a starting powder is high temperature 
processed and thereafter allowed to come in contact with a stream of 
hydrogen gas before cooling. 
BACKGROUND OF THE INVENTION 
Fine spherical powders, especially alloy or elemental metal powders can be 
made by high temperature processing. Such processes are described in U.S. 
Pat. Nos. 3,909,241, 3,974,245, 4,592,781, 4,715,878, 4,502,885, 
4,711,660, and 4,711,661. One of the problems that occurs in this 
processing, especially plasma processing, is that the resulting fine 
spherical powder tends to be high in oxygen levels. This is detrimental 
because presence of oxygen decreases compressibility, sinterability, and 
mechanical properties and electrical condictivity, etc. of the sintered 
compacts. 
It would be a significant advance in the art to assure that spherical 
powder particles which are produced by high temperature processing are low 
in oxygen content to avoid the above described disadvantages. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of this invention, there is provided a 
process for producing fine spherical particles from a starting fine powder 
material which comprises entraining the powder material in a carrier gas, 
passing the powder material and the carrier gas through a high temperature 
zone, and maintaining the powder in the high temperature zone for a 
sufficient time to melt at least about 50% by weight of the particles of 
the powder and to form spherical particles of the melted portion, allowing 
the high temperature treated material to come in contact with a reducing 
atmosphere created by a stream of hydrogen gas, and thereafter rapidly 
solidifying the resulting high temperature treated material to form 
spherical particles wherein the oxygen content of the spherical particles 
is reduced by greater than about 10% by weight from the starting powder. 
DETAILED DESCRIPTION OF THE INVENTION 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above description of some of the aspects of the invention. 
The present invention provides a method for high temperature processing of 
powder to produce spherical particles which are consistently low in oxygen 
content. The spherical particle powders thus produced are more 
compressible, sinterable, have improved mechanical properties, and 
electrical conductivities over the higher oxygen content powders. 
This invention is applicable to any powder material. However, it is 
especially suited to metal powders and metal alloy powders such as 
tungsten metal and alloy powders, iron-cobalt alloys, other iron-based 
alloys, and cobalt metal powders. 
The starting fine powder material is entrained in a carrier gas such as 
argon and passed through the high temperature zone and maintained in the 
high temperature zone at a temperature above the melting point of the 
powder for a sufficient time to melt at least about 50% by weight of the 
powder and form spherical particles of the melted portion. Some additional 
powder particles can be partially melted or melted on the surface and 
these can be spherical particles in addition to the portion of completely 
melted particles. The preferred high temperature zone is a plasma. 
Details of the principles and operation of plasma reactors are well known. 
The plasma has a high temperature zone, but in cross section the 
temperature can vary typically from about 5500.degree. C. to about 
17,000.degree. C. The outer edges are at low temperatures and the inner 
part is at a higher temperature. The retention time depends upon where the 
particles entrained in the carrier gas are injected relative to the nozzle 
of the plasma gun. Thus, if the particles are injected into the outer 
edge, the retention time must be longer, and if they are injected into the 
inner portion, the retention time is shorter. The residence time in the 
plasma flame can be controlled by choosing the point at which the 
particles are injected into the plasma. Residence time in the plasma is a 
function of the physical properties of the plasma gas and the powder 
material itself for a given set of plasma operating conditions and powder 
particles. Larger particles are more easily injected into the plasma while 
smaller particles tend to remain at the outer edge of the plasma jet or 
are deflected away from the plasma jet. 
The resulting high temperature treated material is then allowed to come in 
contact with a reducing atmosphere created by a stream of hydrogen gas. In 
accordance with a preferred embodiment, the hydrogen gas stream exits a 
3/8" spray nozzle such as manufactured by Vortec. The nozzle is positioned 
about 2" down from the powder port which is the point of entrance of the 
powder into the high temperature zone, and at about a 90.degree. angle 
from the direction of the powder port. This creates a reducing atmosphere 
and allows the high temperature treated material which is mainly in the 
form of molten particles to come into contact with the hydrogen gas before 
cooling. 
The oxygen content is variably affected by high temperature processing when 
the contacting with hydrogen step is not done. The oxygen content can 
increase or decrease by varying degrees, depending on the nature of the 
powder. When the oxygen content does decrease, it is typically by no more 
than about 10% by weight over the starting powder. With the step of 
contacting with a hydrogen gas stream, a reduction in oxygen by greater 
than about 10% by weight is realized. 
As an example of the advantages afforded by the present invention, when 
tungsten powder containing about 1270 weight parts per million oxygen is 
plasma processed without the hydrogen gas stream the oxygen level of the 
resulting spherical particles ranges from as high as about 0.49% to as low 
as about 0.20% by weight. When the high temperature treated material is 
contacted with the hydrogen gas stream according to the process of the 
present invention, the oxygen level in the resulting spherical particles 
is as low as about 780 weight parts per million. 
The high temperature treated material is then rapidly solidified. Generally 
the major weight portion of the material is converted to spherical 
particles. Generally greater than about 75% and most typically greater 
than about 85% of the material is converted to spherical particles by the 
high temperature treatment. Nearly 100% conversion to spherical particles 
can be attained. The major portion of the spherical particles are less 
than about 20 micrometers in diameter. The particle size of the plasma 
treated particles is largely dependent on the size of the starting powder 
material. 
After cooling and resolidification, the resulting high temperature treated 
material can be classified to remove the major spheroidized particle 
portion from the essentially non-spheroidized minor portion of particles 
and to obtain the desired particle size. The classification can be done by 
standard techniques such as screening or air classification. The unmelted 
minor portion can then be reprocessed according to the invention to 
convert it to fine spherical particles. 
Spherical particles have an advantage over non-spherical particles in 
injection molding and pressing and sintering operations. The lower surface 
area of spherical particles as opposed to non-spherical particles of 
comparable size, and the flowability of spherical particles makes 
spherical particles easier to mix with binders and easier to dewax. 
To more fully illustrate this invention, the following nonlimiting example 
is presented.

EXAMPLE 
Fine tungsten powder having a mean size of about 2.5 micrometers and a 
starting oxygen content of about 1300 weight parts per million is 
entrained in an argon carrier gas flow of about 7.5 SCFH and passed 
through a high temperature plasma gas. This gas consists of about 80 SCFH 
argon and about 15 SCFH helium. The current is about 400 amps and the 
voltage is about 40, giving a power of about 16 kw. The powder upon 
exiting the flame enters a reducing atmosphere consisting of a hydrogen 
gas stream of approximately 30 SCFH. The material is then rapidly 
solidified and collected. The oxygen content of this material is about 780 
weight parts per million compared to contents of about 2000-4000 weight 
parts per million when the hydrogen is not employed. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.