EP0156629A2

EP0156629A2 - The production of metal particles from molten metal

Info

Publication number: EP0156629A2
Application number: EP85301976A
Authority: EP
Inventors: Charles King
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1984-03-21
Filing date: 1985-03-21
Publication date: 1985-10-02
Also published as: GB8407381D0; EP0156629A3

Abstract

Process and apparatus for producing metal particles (12 or 32) from molten metal (2) using a gas flow to comminate a thin film (11 or 11a) of metal formed on an angled portion (6 or 26) of a surface (5 or 25) wetted by the metal (2). The angled portion (6 or 26) subtends an angle of from 5 to 120°. Preferably any rounding (8) of the edge (7) or point (27) defined by the angle should have a radius of curvature of from 4 to 25µm. Free flowing particles (12 or 32) are obtainable.

Description

This invention relates to a process and apparatus for producing metal particles from molten metal by means of comminution caused by gas flow.
An early process for producing metal particles from molten metal comprised extruding molten metal into cold water. However the metal particles obtained were coarse and of mainly unpredictable shapes and sizes. Metal particles of more predictable shapes and sizes have been made by pouring molten metal onto rapidly moving surfaces usually provided by discs or cups rotating rapidly enough to generate centrifugal forces strong enough to cause comminution of the molten metal. Rotational speeds as high as 10000 revolutions/minute are often needed which means that the apparatus used must be of robust construction and is accordingly expensive. Such processes produce uniformly shaped particles provided the particles have diameters of not less than 30µm. An alternative process which avoids the need to use apparatus having rapidly moving parts comprises subjecting a jet of molten metal to a blast of non-oxidising gas. A problem with this process is that in order to overcome the surface forces which hold the metal in its jet shape, it is usually necessary to use a pressure gradient of at least 20 bar to generate the blast and blasts of this force are expensive to generate and control. For this reason metal particles made by the jet-blasting process are less predictable in shape and size. An object of the present invention is to provide a process and apparatus for producing metal particles from molten metal which are simple to operate and yet able to produce particles which are reasonably pre-determinable in shape and size and which may be smaller than 30pm in diameter if so required.
Accordingly this invention provides a process for producing metal particles from molten metal by means of comminution caused by a flow of non-oxidising gas wherein the comminuting action of the gas flow is assisted by a surface which is wettable by the molten metal and which is shaped to define an angled portion which subtends an angle of from 5 to 120° and the process includes the steps of

a) forming a film of molten metal on the surface, the film being thin enough for surface forces in the molten metal to cause the film to adhere to the surface,
b) advancing molten metal in the film to the angled portion of the surface,
c) contacting the film in the region of the angled portion with the flow of non-oxidising gas, the flow having sufficient kinetic energy to comminute the molten metal at the angled portion into metal particles and
d) conveying the metal particles away from the angled portion at a rate at least equal to the rate at which molten metal is being advanced to the angled portion.

The presence of the angled portion of wettable solid enables the molten metal in the region of the angled portion to be comminuted by much less energetic gas flows. For example the kinetic energy of the gas flow may be such as is generated by a pressure gradient of as little as 1.5 bar. Low energy gas flows are more easily controlled and are better able to produce particles of pre-determinable shapes and sizes.
In practice it is virtually impossible to achieve an angled portion in which the angle is not rounded to some extent. It is preferred that the rounded angle should comprise at least a portion of curvature for which the radius of curvature is not more than 25pm. This ensures that despite the rounding, the angle is still sharp. It has been discovered that the sharpness further reduces the amount of kinetic energy needed to comminute the molten metal. Good sharpness is easier to achieve if the angle subtended by the angled portion does not exceed 95°. Smaller angles and greater sharpness both favour the production of smaller metal particles and so variation of either provides means for pre-determining the size of metal particles produced. The angle may define a point or an edge and the edge may be straight or curved. Points are easier to sharpen but edges allow higher production capacities. Useful curved edges are provided by rims, for example the inside rims of precisely machined gas nozzles.
In theory the upper limit to the sharpness of the angle is set by the point at which the surface meniscus of the molten metal is ruptured and sharpness beyond the rupture point is not desirable. Also in practice very sharp angles may be vulnerable to damage by abrasion and so in general it is not worthwhile attempting to achieve a radius of curvature of below 4pm. For similar reasons it is preferred not to use angles more acute than 2₀₀.
The non-oxidising gas may be a gas which is inert with respect to the molten metal, for example argon or in some cases nitrogen or carbon dioxide or it may be a reducing gas, for example hydrogen or methane. It has been discovered that reducing gases further facilitate comminution of the molten metal. It has also been discovered that the temperature of the gas flow influences the shape of the metal particles produced. Cold gas flows favour the production of elongated particles whereas hotter gas flows favour the production of spherical particles. Accordingly adjustment of the temperature of the gas flow provides means for pre-determining the shape of the particles.
The gas flow must contain sufficient energy to be able to comminute the molten metal at the angled portion of the wettable surface. The amount of energy needed depends not only on the angle subtended by the angled portion and its sharpness, but also on the surface tension of the molten metal. The energy of the gas flow also influences the size of the particles produced with high energies favouring the production of smaller particles. The kinetic energy in the gas flow is most conveniently assessed in terms of the pressure gradient used to generate the flow. For molten metal (for example molten tin or its alloys) having surface tensions below 600mN/m it is preferred to use pressure gradients of from 1.5 to 4 bar. Pressure gradients of from 3 to 8 bar are preferred for metals such as aluminium, copper or gold which have intermediate surface tensions of from 500 to 1300 mN/m. Pressure gradients of above 6 bar are preferred for metals having high surface tensions of above 1200 mN/m, for example chromium, iron, cobalt, nickel, titanium, platinum, palladium or ruthenium. Generally there is no need to use pressure gradients of above 15 bar.
The energy in the gas flow serves another purpose, namely it generates a zone of turbulence around the mainstream of the gas flow. Freshly formed metal particles which are still molten droplets are conveyed away from the angled portion by this zone of turbulent gas and it has been found that the turbulence may be used to minimise if not prevent contact between molten droplets. Therefore, provided the droplets are given adequate time to solidify, the turbulence enables the production of a free flowing powder comprising largely unsintered solid metal particles.
It is essential that the molten metal should wet the surface which defines the angled portion (that is to say the contact angle between the molten metal and the surface should be less than 90°) for otherwise it is impossible to cause the film to adhere to the surface. It is also essential for the film to be thin enough to enable surface forces to cause it to adhere to the surface. A convenient method for forming such a thin wetting film comprises supplying molten metal to the surface via a film-defining slit. Molten metal from the slit is advanced to the angled portion of the surface probably by means of surface forces possibly assisted by pressure within the molten metal. Preferably the film-defining slit is located above the angled portion so that use can be made of a pressure head generated by the weight of the metal. The film attenuates as it leaves the slit and to give ample opportunity for attenuation to occur it is preferred that the distance along the surface from the slit to the angle subtended by the angled portion be at least 5 times the width of the slit. Attenuation is enhanced in the vicinity of angles and especially in the vicinity of sharp angles. Attenuation permits the formation of film as thin as 1 to 30pm. However, attenuation is at least inhibited if the rate at which metal particles are conveyed from the angled portion does not at least equal the rate at which molten metal is being advanced to the angled portion.
Metal particles made by a process according to this invention may be for example spherical, oblate spherical or elongated and the diameters of the spheres or the diameters of a cross-section of the elongated particles may be as small as from 1 to 30pm. Frequently the size of the particles is uniform to the extent that 90X by number of the particles have such diameters which are within 10X of the number average of the diameters. The particles find uses in most applications for metal powders, for example powder metallurgy, pigment compositions, compositions for making electronic circuitry and dental materials. The elongated particles are of particular value to the dental trade because they possess compaction properties which can only be assessed subjectively but which are immediately recognisable by dentists.
This invention also provides apparatus suitable for producing metal particles from molten metal by means of comminution caused by a flow of non-oxidising gas wherein the apparatus comprises

a) a surface wettable by the molten metal and shaped to define an angled portion which subtends an angle of from 5 to 120",
b) means for providing a film of molten metal at the angled portion of the surface and
(c) means for directing the flow of non-oxidising gas into contact with the molten metal at the angled portion.

To facilitate formation of the film of molten metal, the apparatus preferably also comprises a film-defining slit positioned so as to be able to supply a film of molten metal onto the wettable surface. Preferably the slit is located above the angled portion of the surface and metal is advanced by surface forces but may also be such that the weight of the molten metal can be used to create a pressure head.
The invention is further illustrated by the following embodiments which are described with reference to the drawings in which:-

Figure 1 shows a section through an apparatus according to the invention;
Figure 2 shows on a larger scale the tip of the nozzle shown in Figure 1;
Figure 3 shows on a very much larger scale the rounding on the right-angled rim shown in Figure 2, and
Figure 4 shows in partial section a part of an alternative apparatus according to this invention.

Figure 1 shows a tank 1 containing a quantity of molten metal 2 through which passes a gas nozzle 3 which has an internal diameter of 2mm and is made of a metal which is wetted by molten metal 2. Nozzle 3 has a tip 4 which protrudes from tank 1 and provides a surface 5 which is shaped to define an angled portion 6 shown more clearly in Figure 2. Angled portion 6 subtends a right angle and extends circumferentially to define inside rim 7 of tip 4. Rim 7 has a slightly rounded edge 8 as shown in Figure 3 and to ensure the sharpness of rim 7, curved edge 8 has a radius of curvature of only 5pm. The lower part of tank 1 converges to form neck 9 which fits closely around nozzle 3 leaving an annular clearance of 10µm which constitutes film-defining slit 10.
In operation molten metal 2 enters slit 10 where viscous forces permit only a slow controlled flow of metal to reach surface 5 where it forms film 11 which is initially 10µm thick. Surface forces in molten metal 2 causes film 11 to adhere to surface 5 and attenuate. A combination of surface forces and pressure generated by the weight of the metal cause the molten metal in the film to advance to rim 7 where attenuation is extreme. Non-oxidising gas is made to flow down nozzle 3 and to contact film 11 in the region of angle portion 6 with sufficient kinetic energy to comminute the molten metal into particles 12 consisting of droplets of molten metal. The mainstream of the gas flow leaving nozzle 3 is indicated by arrows 13 and is surrounded by a zone of turbulence indicated by arrows 14. Particles 12 are caught up in this zone of turbulence and conveyed away from angled portion 6 with few collisions between adjacent particles 8 so that particles 8 solidify to form a free flowing powder.
Figure 4 shows part of an alternative apparatus in which nozzle 3 is replaced by needle 23 which provides a wettable surface 25 shaped to define a conical angled portion 26 which subtends an angle of 60° which in turn defines a rounded point 27. The radius of curvature of rounded point 27 is 5µm.
In operation, a neck ₉a of the type described with reference to Figure 1 is used to form a thin film lla of molten metal on surface 25. Film lla adheres to surface 25 where it attenuates and advances to angled portion 26 to be contacted with a flow of non-oxidising gas having sufficient kinetic energy to comminute molten metal at point 27. The gas flow is indicated by arrow 33 and is delivered transversely of needle 23 by nozzle 15. Comminuted metal particles 32 are entrained in the zone of turbulence surrounding the gas flow and thereby conveyed away from point 27 and can be collected as a free flowing powder.
- The invention is also illustrated by the following Examples of which Example A is comparative.

EXAMPLES 1 TO 3 AND A

In Examples 1 to 3, the apparatus shown in Figure 1 was used to produce free flowing particles from molten tin or a molten alloy of silver whose composition is shown in the footnote to Table 1. The gas nozzle used was made from copper so providing a surface which was wettable by tin and the alloy. The reservoir of molten metal was maintained under an atmosphere of nitrogen at a pressure adjusted to ensure that the film-defining slit was able to deliver a slow controlled flow of metal. Molten metal at the rim of the nozzle was comminuted by a flow of nitrogen generated by a pressure gradient of 3 bar. The temperature of the metal in the reservoir and the gas in the nozzle are shown in Table 1 together with the nature of the particles produced.
The procedure of Example 1 was repeated for the purposes of Comparative Example A except that the right angled inside rim in the tip of the nozzle was abraded away until the angled portion subtended an angle well in excess of 120^0. As shown in Table 1, it was impossible to obtain particles which were either small or free flowing. Table 1 also shows that hotter gas temperatures favour spherical particles whereas colder gas temperatures favour elongation.

Claims

1. A process for producing metal particles from molten metal by means of comminution by a flow of non-oxidising gas wherein the comminuting action of the gas flow is assisted by a surface which is wettable by the molten metal and which is shaped to define an angled portion which subtends an angle of from 5 to 120° and the process includes the steps of

a) forming a film of molten metal on the surface, the film being thin enough for surfaces forces in the molten metal to cause the film to adhere to the surface,

b) advancing molten metal in the film to the angled portion of the surface

c) contacting the film in the region of the angled portion with the flow of non-oxidising gas, the flow having sufficient kinetic energy to comminute the molten metal at the angled portion into metal particles and

d) conveying the metal particles away from the angled portion at a rate at least equal to the rate at which metal is being advanced to the angled portion.

2. A process according to claim 1 wherein the gas flow is generated by a pressure gradient of from 1.5 to 15 bar.

3. A process according to claim 1 or claim 2 wherein the angled portion subtends a rounded angle comprising a portion of curvature for which the radius of curvature is not more than 25pm.

4. A process according to any one of the preceding claims wherein the non-oxidising gas is a reducing gas.

5. A process according to any one of the preceding claims wherein the film of molten metal is formed with assistance from a slit.

6. A process according to claim 5 wherein the distance from the slit to the angle subtended by the angled portion of the surface is at least five times the width of the slit so as to afford the film an opportunity to attenuate.

7. Apparatus suitable for use in performing a process according to claim 1 wherein the apparatus comprises

a) a surface wettable by the molten metal and shaped to define an angled portion which subtends an angle of 5 to 120°,

b) means for providing a film of molten metal at the angled portion of the surface and

c) means for directing a flow of non-oxidising gas into contact with the molten metal at the angled portion.

8. Apparatus according to claim 7 which comprises a film-defining slit positioned so that the slit can deliver a film of molten metal to the wettable surface.

9. Apparatus according to claim 8 wherein the angle subtended by the angled portion defines the inside rim of a gas nozzle through which the flow of non-oxidising gas passes.

10. Apparatus according to claim 8 wherein the angle subtended by the angled portion defines a point and the apparatus comprises a gas nozzle positioned so as to direct a flow of non-oxidising gas transversely of the angled portion.