CA1061968A - Spheroidization method and apparatus - Google Patents

Abstract

SPHEROIDIZATION METHOD AND APPARATUS

ABSTRACT OF THE DISCLOSURE
In preferred form, the inventive concept utilizes arc-spraying metalizers wherein two metal wires maintained at a difference of potential are fed into an arc gun which positions the wires for contact at a point, such contact generating intense electrical heat sufficient for fusion to be produced as a result of the current flow through the wires at the area of resistance forming the point contact, causing a globule of molten wire to be formed. A jet of non-oxidizing gas is directed towards the contact junction of the wires and causes globules released by the fusing contact to be directed as droplets to a controlled atmosphere chamber wherein the molten metal droplet takes on a spherical shape, cools and solidifies. The chamber wherein the heating process occurs is completely sealed and filled with inert gas. In addition, the inner walls of the chamber are provided with a specific liquid wash curtain preventing metal particles from sticking to the chamber walls.

Description

6~L96~3 i ~_-218 SPHEROIDIZATIO~I METHOD AND APPARATUS
DMR: fdc . . . This invention relates to a method of manuracturing small spheres and particularly to a method for making metal spheres of uniform size and shape with a relatively high yield ~actor.
- Rounded metal particles find general commercial utility in various processes, devices and apparatus. One particular use for such particles is as a carrier or toner in an - electrostatographic reproduction device. Spheroidiæation .techniques for producing rounded metal particles are generally 10 known in the industry. One early technique, known as the metal spraying method, involves the atomizing of metallic wires in the presence of a flame by means of a jet of compressed - . air~ Particle uniformity, production rate and yi~eld from such a technique left much to be desired. A reflnement of the .~15 fore~oing techniqua, known as the arc-spraying method, involves .- . . the simultaneous fusing and atomizing of two conductive, current carrying wires producing an arc on contact with each other~ This technique employs a feed mechanism for providing - .. an infinitely variable continuous feed of both wires às required as well as a contact means for transmltting current to the ... .wires. Also, suitable a-omizing means is positioned adjacent the wires for blowing the fused metal particles away from the contact or.fusing area. The atomizing means conventionally - takes the orm of a jet of compressed air which serves to remove .25 the fused metal particles. The removed metal particles are directed toward a collec~ion device which includes some ~orm

- 2 - ..

.

.
lV~i196~

of collection medium which may also serve a cooling function.
The arc spraylng technique, while improving the yie~d factor xelative to the flame spraying method, still leaves much to be desired in terms of yields, particle roundness, and particle density. An improved version of f:Lame fusion is shown in U.S.
patent ~,269,528 to GallupO Here, a flame fusion method of manufacturing small metal spheres is disclosed wherein the flame fused molten metal is caused to fall through space onto a resilient material, breaking the metal into smaller metallic particles which are then solidified and cooled. The randomness of this operation, however, results in a yield factor as well as roundness and density factors which also leave room for improvement.
One important factor which has been found to - 15 affect the effectiveness of a spheroidization process is ; ~ oxidation. By insuring against oxidation of the molten metal, the ultimate particle will have improved characteristics in terms of uniformity, roundness and density, thereby improving the yield factor. In most prior art devices, however, oxidation is in~erent in the operation of the droplet - ormation and cooling process. In the aforementioned .
Gallup patent, a non-oxidizing chamber eonveys the particles - ~rom the flame area to the resilient contact surface. However, while ~eduction of oxidation during cooling is advantageous in modifying the extent of oxidation, the particle does encounter oxygen during its flame formative process. In addition, the particles in striking the contact surface suffer in reduction of roundness and-uniformity.

.

619~
It is, therefore, an object of an aspect of the present invention to provide a novel and unique method and apparatus for manufacturing particles or spheres of acceptable roundness and density with a yield heretofore higher than that achieved by prior art processes.
It is an object of an aspect of the present invention to utilize an arc fusion technique for the manu-facture of metallic particles which will provide processing rates higher than heretofore attainable.
It is an object of an aspect of the present invention to provide a method and apparatus for manufacturing metallic particles with high yields which substantially reduces or controls the degree of oxidation.
It is an object of an aspect of the present invention to utilize an arc spraying technique to form metallic particles in a continuous or discontinuous process with substantially reduced oxidation and with yields higher than heretofore attainable.
It has been discovered that a major cause of oxidation during particle formation is the presence of oxygen in the jet of compressed air conventionally employed to atomize the globules formed at the fusion area. In accordance with one aspect thereof, the present invention employs method and apparatus for melting a source of material to form a successive plurality of molten globules, and directing a flow of an inert or non-oxidizing compressed gas against the area of contact, causing the globules to be atomized and propelled away from the molten area as droplets into a non-oxidizing cooling chamber, wherein the droplets cool and solidify into particles.
In preferred form, the inventive concept utilizes arc-spraying metalizers wherein two metal wires maintained 1-~

`` ~0619~
at a difference of potential are fed into an arc gun which positions the wires for contact at a point, such contact generating intense electrical heat sufficient for fusion to be produced as a result of the current flow through the wires at the area of resistance forming t:he point contact, causing a globule of molten wire to be formed. A jet of non-oxidizing gas is directed towards the contact junction of the wires and causes globules released by the fusing contact to be directed as droplets to a controlled atmosphere chamber wherein the molten metal droplet takes on a spherical shape, cools and solidifies. The chamber wherein the heating process occurs is substantially sealed and filled with inert gas. In addition, the inner walls of the chamber are provided with a specific liquid wash curtain preventing metal particles from sticking to the chamber walls.
In accordance with one aspect of this invention there is provided a method of manufacturing particles comprising the steps of meltins a source material in the substantial absence of o~ygen to form molten globules at a point, said source material being provided by feeding first and second metal wires from first and second sources to said point, providing ; sufficient electrical current flow along said wires through said point to create fusion and cause globules of molten wire to be formed, said globules being formed from both of said wires, directing a flow of an inert non-oxidizing compressed gas toward said point against said globules to atomize globules into droplets and propel said droplets away from said point into a chamber where said droplets cool and solidify into particles.
In accordance with another aspect of this invention there is provided an apparatus for manufacturing metal particles comprising a walled chamber, means for feeding at least a first and second wire to a common point within said ~ _5_ C

)6196~3 chamber, a power source coupled to said first and second wires for providing a current flow through sa:id wires and said common point, said current flow sufficient to generate heat at said common point causing said metal to globulize at said point and form a globule of molten meta:L thereon, a substan-~ -Sa-.f~

96~3 tially non-oxidizing gas, means for directing a flow of said substantially n~n-oxidizing gas from a source of said gas toward said common point at a rate sufficient to dislodge and atomize said globule into droplets and envelope said droplets into a substantially non-oxidiz:ing environment, said flow propelling said droplets into said chamber, said atomized globule droplets corresponding to said particles.
In accordance with another aspect of this invention there is provided an apparatus for manufacturing metal particles comprising a walled spheroidization chamber and a walled collection chamber, said spheroidization chamber and said collection chamber each sealed in an air tight manner, a plurality of wire sources, means for feeding wire from each source into said spheroidization chamber, a plurality of wire guides mounted to said spheroidization chamber, each of said wire guides positioned with respect to each other for permitting passage of a wire thexein to a common point formed by the intersection of said plurality of wires within said spheroidization chamber, a power supply coupled to said wire guides for providing a current flow along said plurality of wires through said common point, said current flow through said common point sufficient to generate heat causing said wires to gl.obulize, forming a globule of molten metal at said -point, a source of inert non-oxidizing compressed gas, means for directing a flow of said inert gas from said source of gas into said spheroidization chamber toward said common point, said flow of gas sufficient to dislodye said globule and direct the resultant droplet corresponding to one of said particles along a path toward said collection chamber; said droplet cooling and solidifying along said path, wash means, a recycle conduit coupling said wash means to said spheroid-ization chamber, said wash means forcing a wash liquid through : ^~
~ -5a-1~6~96~

said conduit into said spher~.idization chamber in a manner .
causing said li~uid to coat the inner portion of the spheroid-ization chamber wall which is lying in said path, and an exit port positioned in said collection chamber for removing said particles from said collection chamber.
The foregoing object and brlef description, as well as further objects, advantages, and features of the present invention will become more apparent from the following more detailed description and appended drawings wherein:
Figure 1 is a generalized schematic diagram of the two wire process; and Figure 2 illustrates the internal operation of a spheroidization column.

519c ., . ~ . .

1~6196~3 Referring to the general process schematic in Figure 1 the spheroidization column 10 is fitted with an arc spheroid-i~ation assembly 12 which-includes the metallizing gun. The spheroidization column and more specifically the metallizing gun is provided with a sequentially fed wire set 14 including -a first wire 16 and a second wire 18, each being fed from the wire supply units ~0 and 22 respectively. The wires may be fed by an electrical drive or an air pressure drive device.
Other drive devices may obviously be employed. The wires are fed into the assembly area 12 and then down into the spheroidization column as will be explained in further detail below. In accordance with the process, a source of an inert non-oxidizing gas 26 is fed through line 28 by means of a high pressure supply or the like, not shown/ into the top ~- portion of the arc spheroidization assembly 24. The wire elements 16 and 18 are energized by leads 32 and 34 derived from the power supply indicated generally as 36.
` A washing-recycling system indicated generally as 38 is provided with an inlet conduit 40 providing an internal wash curtain to the interior walls of the spheroidization column. The wash effluent exits the spheroidization column ~ia conduit 42 into the supply reservoir 43, and is recycled back into the column by the recycling pump 41 along line 40.
The supply reservoir 43 may include filtration or like quality replenishment of the liquid. When metallization occurs inside the spheroidization column, the resultant product effluent is recovered, either as particles alone or in the form of a slurry, via the output conduit 44 from the lower portion of the spheroidization column. The particles thus provided are discharged into a drying unit 46, and then to a screening unit 48 which may serve the function of classifying ~619~i8 particles in accordance with a desired mesh size, and placing the desired par-ticles in the receptacles 50 and the undesired particles in the scrap unit 52.
Referring now to Figure 2, the interior of the spheroidization column is shown in greater detail. As is evident, the spheroidization process utilizes arc fusion.
The wiresr illustrated again with reference numerals 16 and 18, are advanced by a suitable feed mechanism not shown. A set of wire sleeve guides 53 and 54, which are, for purposes of this illustration, shown as hollow conductors mounted into the insulating upper surface 56 of the spheroidization column.
Power in the form of a potential difference is applied to the wires 16 and 18, by a flow of current supplied from the electrical leads 3? and 34 to the conductive contacting wire sleeve guides 53 and 54.
An atomi~ing nozzle 30 is mounted into the insulat-ing upper portion 56 and provides for an adjustable flow of a non-oxidizing gaseous stream to the point of intersection of the two wires 16 and 18. As the wires are advanced by the feed mechanism, they touch at a common point. Due to the high density of the current and the contact point resistance, the extreme heat generated results in an arc at the contact surface and also results in fusion of the contacting areas of the metal wires. The melted portion thus forms a globulized portion 58. The inert, non-oxidizing gaseous s-team is pro-vided at a momentum sufficient to remove and a-tomize the globulized portion 58 from the contact area in the form of droplets which correspond to the particles ultimately formed.
Removal of a globule causes the arc to be briefly in-terrupted.
The resulting space is quickly filled by advancing feed mechanism, and the process repeats. The proper conditions `` 1~619~
for the regular burning of the arc are created by adjusting the flow rate and pressure of the gas, the voltage and current, and by regulating the wire feed rate. By way of example, a typical current value can range in the order of several I hundred amperes. The angle at which the two wires are fed ; toward each other can also be adjusted for maximum efficiency of yield.
The plurality of particles thus formed, indicated generally as 60, fall away from the arc region of the inter-secting wires 16 and 18 as droplets and down into the collect-ing pan 62. As they fall, they are cooled and solidify into spherical ~orm. The collecting pan area serves to cool as well as collect particles. From the collecting pan, the particles are forced out along the output conduit 44, either in the form of a slurry, by an auger feed, gravity, or other - -- suitable means for removal~ If the conduit is large enough, the particles could be collected, dried and discharged through a cooling auger or other suitable device.
To further improve the characteristics of the ~0 spheroid particles thus formed, a wash recycle system 38 is utilized. Thus, the inlet conduit 40 provides a wash material curtain mechanism which prevents particulate material 60 from sticking to the sldes of the spheroidization column. The inlet 40 provides a stream of wash material, preferably a liquid, into what is shown as preferably a circular perforated distributar, ~hich may be formed as a ring 64 mounted high on the interior portion of the spheroidization column. The ring 64 may include a plurali-ty of ou-tlets 66 which result in placement of the wash liquid along the interior portion of the wall of the spheroidization col~n. The ring urlit 64 and outlets 66 may be positioned or aligned in a manner such tha-t ~ ~19~

the flow o~ liquid emerging from the outlet 66 follc~ws a down-ward pat-tern about the interior o.~ the wall 6~, flowing down the inside of the colleeting pan to an exi.t area. The liquid exits the area by -the exit eondui-t 42 positioned so as to maintain a desired level of liquid in the eollecting pan 62 and/or spheroidization column 10. As illustrated, the position of the ring 64 is sueh tnat it assures that a liquid eurtain e~ists along any portion of the wall which will be, or possibly may be, struek by particles ejected from the arc portion of the wire intersection. The general configuration of the particle discharge from the intersection of the arc portion will be in the form of an inver-ted cone having slightly eireular sides. In this manner, the cone portion defines a perimeter of eontact indicated generally as beginning at the point 70, the highest point along the interior of the spheroidization eolumn or collecting pan whieh is possibly subjeet to being struck by particles 60. sy positioning the ring 64 above the circumference defined by the perimeter of eontact, it will be insured that the liquid eurtain will be on the critical interior wall portion 68 of the spheroidization eolumn and eolleeting pan 62, thus preventing partieles 60 : from striking and stieking to the interior of the eolumn wall. The liquid eurtain also serves to reduce -the impaet forees on partieles striking the interior walls, thereby improving roundness and thus raising yield.
The liquid may be removed from the chamber in several ways. For example, an outlet 42 may be provided above the bottom of the colleeting pan as illustrated in Figure 2.
In this manner, the process may be continuous in the sense that particles will continue to exit the chamber with some liquid as a slurry through the outlet 44 and -that the liquid level seal will be maintained along or above point 45.
As will be evident Erom the descriptions oE Figure 1 and Figure 2, the employment of non-oxidizing environments or a substantiall~ non-oxidizing environment which forms an envelope about the particle from the moment of formation of the particle untilits removal from the collecting pan requires that the spheroidization column be sealed with a controlled atmospherej preferably without oxygen. To this end, an exhaust port, indicated in Figure 1 as element 72, is provided with a suitable means (not shown) such as a back pressure valve or the like which will allow release of gases while maintaining a sufficiently high absolute pressure in the column resulting from the continuous feeding of the non-o~idizing gas into the chamber through the nozzle 30. In this manner, the spheroidization column may continuously be vented without the fear of backwash resulting in oxidation, contamination from air or -the like. Further to this end, the wash recycle system employs a liquid, which is sufficiently inert and substantially non-oxidizing under the condi.tions of use ~)63L96~

.
~ithin the spheroidization col~n, such as a pure water, dilute methanol, alcohol, or other sufficielltly inert liquids. The ~e~p!~rat~re within the collection zone can range from ambient to as high as the boiling points of the specific li~uids. To this end, tne port 44 may also include a restxiction valve or liquid seal such that the differential pressure built up through the nozzle 30 will prevent backwash of oxidizing atmosphere from entering the chamber through the port during removal. Alter-natively, when the removal is discontinuous, the process may be halted during removal of the particles. - -The spheroidization column is of sufficient heightand diameter to permit the atomized droplets to assume their spheroidal shape before striking the collection pan. It has -been found experimentally that good results are obtained in a column having a three-foot internal diàmeter and an eight-foot height. However, it has been found that satisfactory results - with varying yields may be obtained from a height ranging from six inches to about twenty feet, and it is estimated that a - diameter one foot or greater will sufflce.
It is preferred that the column wall be jacketed with suitable interwall circulation of coolants or the like to provide cooling for the wash liquid and to provide further .insurance against sticking.
The factors generally affecting the process yleld .
include, in order of relative importance, the nature of the gas, the gas flow rate, gas pressure, current flo~, and wire intersection angles. As noted above,the gas should preferably be non-oxidizing or at least substantially non-oxidi~ing and , . . .

. ~ ... ~ ... ..

should be inert with respect to the materials utilized to ~orm the particles. It has been found that nitrogen is a suitable gas ~or this purpose, and is preferred.
In a spherodization process, employing steel wire souxce material, .094 inches in diameter, it has been ~ound that acceptable results, providin~ yields significantly higher than obtainable with prior art processes, are achieved with a gas ~low rate ranging between 70 and 100 cubic feet per minute, under a pressure ranging between 80 and 120 pounds per square inch, with a wire intersection angle ranging between 30 and 60 degrees, and an electrical current f~ow of between 300 and 1000 amperes. The wire feed rate may vary between 30 to 180 pounds per hour. A preferred wash liquid employed is a 5~ solution of methanol in water.
15By way of specific example, the spheroidization column described in Figure 2 was emplo~ed to produce metallic ~ particles.
- - EXAMPLE I
. .
. ~ , . . .
- -~ The low carbon steel wire was fed at a rate of 32 lbs/
hour under a current of 350 amps provided by a potential of 43 volts and under a gas pressure of 100 psig. The size yield was 65~ and ~uality rounds 93%. ~ :
EXAMPLE II
The low carbon steel wire ~as fed at a rate of 25 - 46 lbs/hour under a current o~ 525 amps provided by a potential of 45 volts under a gas pressure of 90 psig. The size y~eld was 61% and quality rounds 92~.

.

.. . .
~ . . .

:~116~L~61~
EX~MPLE III
The low carbon steel wire was ~ed at a rate o~ 100 lbs/hour under a current of 750 amps provided by a potential of 40 volts under a gas pressure of ~0 psig. The size yield was 71% and quality rounds 92%.

In the foregoing examples, the size yield indiçates the desired size range achieved compared to total product produced. Quality rounds, determined by the vibrating table - method, indicates the minimum sphere number of acceptable uni~ormi~y. -- - The particles produced in all three foregoing examples were found to have a substantially uniform density indicating both reliability of the process and a low quantity of voids and oxide contamination. ~n addition, the yield and roundness 15-~ percentages are significantly higher than heretofore achieved in-prior art processes.

, The metal particles formed were then employed in a ; ~ skandard xerographic reproduction apparatus. A developer composition was formed, utilizing ~he metal particles as a core material, with a pigmented toner of conventional composition.
-The resultant reproauctions showed high image density andlow background levels, thereby indicating good to excellent ~erographic response. - - : - - -The particular configuration of the metallizing gun ha~ing a separate atomizing nozzle is intended as exemplary - only and is not intended to be limiting. Thus, for example, a further embodiment may be employed wherein the atomiæing '. . . .
~ 13 nozzle surrounds the wire guide sleeves and a pressure is supplied about the exterior rather than toward the interior of the wire guides. Alternatively, it may ~e possible to provide an atomizing nozzle in each of the wire guides and inject the inert non-oxidizing gas thereby to the intersection of the wires metallized.
- In addition, magnetic fields may be employed to contain or stabilize the arcl as by a magnetic pinch effect.
With regard to the gaseous medium employed, although the preferred embodiment employs a reservoir tank of nitrogen, - it is also possible to employ argon, helium, carbon dioxide, or combinations of these or other inert, non-oxidizing gases.
- Heat sufficient to render the wire contact point molten may be supplied by means of any suitable electrical power source ~e.g. motor driven DC generator or constant voltage rectifier, etc.) that provides the potential to cause current flow.

. . . :, he two wire gun illustrated may be replaced by three,~four or more pluralities o~ wire guns. Various com-posi~ions of wire may also be employed through each sleeveto produce other metallic or pseudo-alloy core materials.
While the invention has been described with , -reference to specific preferred embodiments, it will be ~ - apparent to those skilled in the art that various substitutions, ;~ ~ 25~ alternations and modi~ications may be made therein without departing from the spirit and scope of the invention.

- . . .
' ~ . '' '', '. ' . . ''' ' ' .: ' ' , . .

.
l~i ''

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of manufacturing particles comprising the steps of melting a source material in the substantial absence of oxygen to form molten globules at a point, said source material being provided by feeding first and second metal wires from first and second sources to said point, providing suffic-ient electrical current flow along said wires through said point to create fusion and cause globules of molten wire to be formed, said globules being formed from both of said wires, directing a flow of an inert non-oxidizing compressed gas toward said point against said globules to atomize globules into droplets and propel said droplets away from said point into a chamber where said droplets cool and solidify into particles.

2. The method of Claim 1 further including the step of continuously washing the interior wall of said chamber.

3. An apparatus for manufacturing metal particles comprising a walled chamber, means for feeding at least a first and second wire to a common point within said chamber, a power source coupled to said first and second wires for providing a current flow through said wires and said common point, said current flow sufficient to generate heat at said common point causing said wires to globulize at said point and form a globule of molten metal thereon, a substantially non-oxidizing gas, means for directing a flow of said substantially non-oxidizing gas from a source of said gas toward said common point at a rate sufficient to dislodge and atomize said globule into droplets and envelope said droplets into a substantially non-oxidizing environment, said flow propelling said droplets into said chamber, said atomized globule droplets corresponding to said particles.

4. The apparatus of Claim 3 further including wash means, and conduit means coupling said wash means to said chamber for maintaining a continuous flow of wash material along the inside portion of said chamber wall in the path of said droplets.

5. The apparatus of Claim 4 wherein said wash means re-cycles said wash material, including a further conduit form-ing a return path for said material from said chamber to said wash means.

6. The apparatus of Claim 3 wherein said chamber is sealed in an airtight manner.

7. The apparatus of Claim 4 wherein said conduit means includes a ring distributor positioned about the interior wall of said chamber at a point higher than the path of said droplets for providing said continuous flow of said wash material.

g. The combination of Claim 7 wherein said ring distributor provides a downward wash material flow.

9. The combination of Claim 4 wherein said wash material is a substantially non-oxidizing liquid.

10. An apparatus for manufacturing metal particles com-prising a walled spheroidization chamber and a wall collection chamber, said spheroidization chamber and said collection chamber each sealed in an airtight manner, a plurality of wire sources, means for feeding wire from each source into said spheroidization chamber, a plurality of wire guides mount-ed to said spheroidization chamber, each of said wire guides positioned with respect to each other for permitting passage of a wire therein to a common point formed by the intersection of said plurality of wires within said spheroidization chamber, a power supply coupled to said wire guides for providing a current flow along said plurality of wires through said common point, said current flow through said common point sufficient to generate heat causing said wires to globulize, forming a globule of molten metal at said point, a source of inert non-oxidizing compressed gas, means for directing a flow of said inert gas from said source of gas into said spheroidization chamber toward said common point, said flow of gas sufficient to dislodge said globule and direct the resultant droplet corresponding to one of said particles along a path toward said collection chamber; said droplet cooling and solidifying along said path, wash means, a recycle conduit coupling said wash means to said spheroidization chamber, said wash means forcing a wash liquid through said conduit into said spheroid-ization chamber in a manner causing said liquid to coat the inner portion of the spheroidization chamber wall which is lying in said path, and an exit port positioned in said collect-ion chamber for removing said particles from said collection chamber.

11. The apparatus of Claim 10 wherein said wash means recycles said wash liquid, said wash means includes a reservoir and control apparatus for recycling said wash material through said reservoir.

12. The apparatus of Claim 10 wherein said recycle con-duit includes a ring distributor positioned about the interior wall of said spheroidization chamber at a point higher than the path of said droplet for providing a continuous flow of said wash liquid.

13. The combination of Claim 12 wherein said ring distributor provides a downward wash liquid flow.

14. The combination of Claim 13 wherein said wash liquid is a substantially non-oxidizing liquid.