US2038251A - Process for the thermic treatment of small particles - Google Patents

Description

April 21, 1936. VOGT 2,038,251

PROCESS FOR THE THERMIC TREATMENT OF SMALL PARTICLES Filed Jan. 31, 1934 Jnvan v Patented Apr; 21, 1936 UNITED STATES PROCESS FOR THE THERMIC TREATMENT OF SMALL PARTICLES Hans Vogt, Berlin-Dahlem, Germany Application January 31, 1934, Serial No. 709,239 In January 3, 1933 9 Claims. (01. 1481) Small particles of spherical shape are required for various technical purposes. For example in the electro-technical industry magnetic particles of spherical shape are required for the production 01 cores for Pupin coils and recently of cores for high frequency purposes. Experiments have shown that, for magnetic purposes, an extraordinarily dense arrangement of the particles is desirable in order to obtain a high magnetic conductivity. This problem can be best solved, if

the particles are of spherical shape and if spheres or balls of difierent diameters are mixed. The insulation of the particles, which is of primary importance especially in the construction of cores for Pupin coils and to an even greater extent in the production oi cores for high frequency coils, can be effected most easily with spherical particles. The spherical shape is in this instance desirable for three reasons:

so l-because a spherical surface is easiest to coat with an insulating layer.

2-beeause the spherical shape gives the largest average contacting surface and consequently the lowest average surface pressure, so that piercing 25 or rubbing through of the insulating layer is prevented.

3because the densest arrangement and therefore the mam'mum heaping weight and the highest magnetic conductivity are attained by the 30 spherical shape.

The production of such spherical particles has not been successful with the known methods of mechanical disintegration and the reduction of the particles from their oxides because in this 35 instance the shape of the particles is too irregular.

The problem is solved by the process according to the invention.

The fundamental idea of the invention consists 40 in heating the particles indirectly, for instance by the heat radiated from the heated walls of the heating chamber or heating pipe or by producing a. high frequency magnet field or by heated gas, while they are freely floating through the 45 heating zone. The particles thus are melting to little balls. As the particles are indirectly heated and notcoming into direct contact with the combustion gas, they can be very accurately and cleanly treated and it is easily possible to accu- 50 rately govern all conditions regarding temperature and chemical composition of the gas filling the heating chamber.

A specific application of the process according to the invention consists in producing spherical 55 magnetic particles which are required to make magnetic materials for higher frequencies consisting of magnetic particles and insulating films between them to prevent eddy current. The initial material in this case may be cheap magnetic powder material of irregular particle shape or 5 even metal oxide, which is reduced during the process to metal and melted to little balls. Moreover the particles in the same operation can be insulated either by slightly oxidizing the surface in passing them through a zone of oxidizing gas 10 or by adding to the magnetic powder a fusible insulating powder which will melt and form an insulating skin on the magnetic particle after cooling down.

This process of producing magnetic particles 15 will be more particularly described hereafter by reference to the accompanying drawing but it is understood that other applications or embodiments of the fundamental idea are also lying within the scope of the invention.

Fig. 1 shows the conversion of the single particle in difierent phases.

Big. 2 shows a particle coated with an oxide layer or with a layer of insulating materiai.

Fig. 3 shows an apparatus for carrying out the process.

Fig. 3c shows a modified means of supplying heat to the particles.

Fig. 4 shows a practical construction of a furnace for carrying out the process.

Fig. 4a shows the lower end of the furnace.

Fig. 4b shows a modified form of construction of the lower end of the furnace.

In Fig. 1 the porous irregularly shaped metal or metal oxide is designated by la, the gradual melting andreducing of the particle by lb-le, and the absolutely spherical metallic final product by If. If the surface of the particles is oxidized or covered with an insulating coating of another insulating material, a particle as shown in Fig. 2 will result, having an insulating skin According to Fig. 3 which shows a simple fundamental arrangement to realize the process the metal powder a is supplied through a jigging sieve b into a vertical pipe 0 made of refractory material, which is heated from the outer side by a heating device (1. The size of the particles supplied and consequently the particle size of the final product is determined by the mesh width. The particles drop singly through the pipe and are caught at the lower end thereof in a tray l. During their free fall the particles are subjected to the heat effect of a source of heat owing to the heat radiated by the pipe and to theheatconductedbythegasfiillingthe interior of the pipe. The melting process can be controlled according to the selection of temperature and the falling speed of the particles, which can be influencedby the density of the gas in the interior of the pipe-and by the speed and direction of movement of the gas.in the interior of the pipe. For instance the particles may be kept floating in a gas current, flowing vertically upwards. Further by the selection of the gas filling the interior of the pipe it is possible to oxidize the particles or to leave them chemically uninfluenced during the falling, or to successively eflect for example a reduction of the metallic oxide delivered to metal and subsequently an oxidation of the surface of the metal particles by arranging several atmospheres of a certain chemical composition differing from that of the normal air. The melted particles, owing to the surface stresses, will adopt a spherical shape and, if these liquid balls are allowed to solidify during further free fall, a powder is produced composed of absolutely spherical particles with smooth, metallically pure or oxidized surface and of diameters from 0.001 to 0.5 mma, and if these particles are caught for example in an oil bath heated to a suitable temperature, it is possible to harden the-particles which may improve their magnetic characteristics. If a mixture of different substances is fed into the upper end of the pipe, for example a. mixture of metallic powders or metallic oxide powders, itis possible to alloy the metal particles during their fall, the alloy being obtained in pulverous spherical form, and the composition of the final particles representing an alloy of the initial materials being charged. If on the other hand a mixture of a metallic powder or metallic oxide powder and a silicate, such as glass or another material, which is considerably lighter than the first material, is charged, the metal particles are coated with a glass layer during their fall, and consequently eilectively insulated. Another possibility of insulation consists in subjecting the metal particles, which have already once passed through such a furnace and been melted into spherical shape, a second time to the same process, covering the spherical particles with a silicate paste, and ca ying out the process in such a manner that only the silicate melts and coats the metal baJl with an insulating film p as shown in Fig. 2. The same process can be employed if particles of a heavy metal, such as iron, are to coat with a layer of a diiferent metal, for instance in order to effect a better adhesion of the insulating layer to be subsequently applied in a similar manner.

In Fig. 3a the refractory pipe 0 is in the interior of a high frequency coil n which together from Fig. 4, showing a furnace for realizing the process. The irregular powder it passes from a funnel-shaped magazine 9 through a sieve b or several sieves b arranged in series and vibrated by means of a motor q into the pipe c. This pipe is heated by an electric resistance heating on its outer side by an insulating jacket h.

11, the heated portion of the pipe being insulated Above and below the heated zone a cooling device i, or i: is provided consisting of a cover, in which cold air, cold water or the like is circulating. A gas, such as hydrogen, may be allowed to flow into the pipe 0 through a pipe 1:, so that the particles passing through the pipe 0 are reduced or oxidation thereof is prevented. After the.

particles have left the heated zone, they pass into the cooling zone 1': in which, due to cooling of the furnace walls from outside and/or blowing in of cold gas the particles are cooled down, and then drop, further cooling during their fall, into a dish 1 which is also still in the hydrogen atmosphere. The lower portion of the furnace may be constructed as shown in Fig. 41:, that is the catching dish and the discharge pasage for the particles are no longer in the hydrogen atmosphere but in ordinary air atmosphere or in some other oxidizing atmosphere, so that the surface of the particles is oxidized.

A further form of construction of the lower portion of the furnace is illustrated in Fig. 4b. The particles drop directly from the heating zone into an oil bath m where they are quenched and consequently hardened. Instead of sifting the particles into the furnace, they may as well be fed by blowing them in, so as to obtain a substantially homogeneous distribution of the particles over the cross section of the furnace.

The entire process can be conducted continuously in a simple manner by continually or periodically charging the initial material at the top and removing the finished product at the:

bottom.

The above described process represents a novel manner of treating small particles, metallic powders, and enables the simple and cheap production of metal powders the particles of which are of spherical shape and suitable structure. As mentioned the process particularly enables the production, in a simple and cheap manner, of magnetic metal powders, such as are required for the .manufacture of Pupin cores and high frequency coil cores, such powders easily meeting all requirements as regards purity of metal, spherical shape and insulated surface.

With the aid of the process the metal powder or the metal powder alloy can be produced from metal oxides which can be obtained cheaply anywhere, and to thus do away with the necessity.

of employing the processes hitherto employed for this purpose, for example the iron carbonyl process, which is very expensive and also does not produce accurately spherical particles.

It is evident that the constructions and uses above set forth are only examples, and that the process is capable of being generally used.

For instance, the process according to the invention may be applied to any fusible pulverous material of irregular particle shape such as metal, metal oxide, ceramic substances, carbon, silicates, glass or the like or mixtures of any these substances. The particles may in the same Operation be subjected to any thermic treatment such as reducing, oxidizing, annealing, quenching, hardening, alloying or covering of the spherical particles with a layer of another substance.

1. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping said in regular particles through a vertical heating chamber, melting said particles by transmitting heating energy from a source of heat outside the inner walls of said chamber and allowing said melted particles to solidify while flying in a gaseous atmosphere.

2. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping said irregular particles through a vertical heating chamber, in melting .said particles by transmitting heating energy from a source of heat outside the inner walls of said chamber, and in delivering said particles into a liquid.

3. A process for changing discrete fusible powder particles of irregular shape into particles of! spherical shape, consisting in dropping irregular particles through a heating zone in a non-oxidizing gaseous atmosphere and in melting them while they are flying through said zone, without using combustion heat in said chamber. 4

4. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping said irregular particles through a heating zone in a nonoxidizing gaseous atmosphere and in chemically treating and melting them while they are flying through said zone, without using combustion heat in said chamber.

5. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping irregular particles through a heating zone in a hydrogen atmosphere and in melting them while they are flying through said zone, without using combustion heat in said chamber.

6. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping said irregular particles through a heating zone in a hydrogen atmosphere and in chemically treating and melting them while they are flying through said zone, without using combustion heat in said chamber.

7. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping irregular particles through a heating chamber filled with a non-oxidizing gas and melting said particles by the heat radiated by the walls of said heating chamber.

8. A process for changing discrete fusible powder particles of irregular shape into particles of spherical shape, consisting in dropping irregular particles through a heating chamber and melting said particles by a high frequency field produced in the interior of the pipe by a coil wound

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