US 3272615 A
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p 1966 D. J. N. HOFFMAN ETAL 39 9 PRODUCTION 0F SPHEROIDIZED PARTICLES Filed July 25, 1963 United States Patent 3,272,615 PRODUCTION OF SPHEROIDIZED PARTICLES Daniel l. N. Hoffman and Thomas ll. Beeton, Pretoria, Transvaal, Republic of South Africa, assignors to South African Iron and Steel Industrial Corporation Limited Filed July 25, 1963, Ser. No. 297,635 Claims priority, application Republic of South Africa, Aug. 1, 1962, 626,266 2 Elaims. (Cl. 75.5)
This invention relates to the production of spheroidized particles, and more particularly spheroidized ferrosilicon particles.
The specification of our US. Pat. 3,015,852, discloses a method of spheroidizing irregularly shaped solid particles, such as ferro-silicon particles, to form smooth spheroidized solid particles by passing the particles through a high temperature flame in which they are melted at least at their surfaces, and thereafter cooling the thus treated particles.
More particularly, a method of spheroidizing irregularly shaped particles includes the steps of passing a gas containing free oxygen in at least the proportion contained in air into an inner passage of a flame-producing nozzle; introducing a combustible gas through an annular passage in the nozzle surrounding the inner passage, thus producing a flame having a reducing zone at least towards its perimeter; feeding the irregularly shaped particles to be spheroidized into the inner passage; causing the particles to pass through the flame and its reducing zone thereby melting them at least at their surfaces; and allowing the thus spheroidized particles to enter a cooling zone.
With the method defined in the previous paragraph, oxidizing gas is blown into the center of the flame, and a combustible gas is supplied to produce a surrounding reducing zone at least towards the periphery of the flame. Particles to be spheroidized pass from the oxidizing center of the flame through the reducing zone upon leaving the flame and, as a result, too extensive oxidation of the particles is prevented. A so-called inverse flame is produced.
It is believed that the flame characteristics can be improvide by mixing a small proportion of combustible gas with the oxidizing gas introduced into the inner passage of the flame-producing nozzle.
Preferably, the irregularly shaped particles to be spheroidized are passed through a downwardly directed pencil-shaped flame.
irregularly shaped ferro-silicon particles containing from about to silicon, and preferably from about 12 to 17% silicon, are very suitable as the initial material. Small amounts of other alloying constituents, such as, for example, copper or aluminum, which may have a beneficial effect on the spheroidizing, corrosion resistance, or other qualities of the particles, may be present or incorporated therein. The initial material may, for example, be mechanically ground and then subjected to the spheroidizing treatment.
It is an object of the present invention to provide improved spheroidized particles which are superior to the particles obtained in accordance with the method disclosed in our US. Patent 3,015,852.
According to the invention a method of spheroidizing irregularly shaped particles includes the steps of providing a high temperature flame; imparting a swirling motion to the irregularly shaped particles; passing the swirling particles through the flame; and allowing the particles to pass from the flame into a cooling zone.
More particularly, a method of spheroidizing irregularly shaped particles includes the steps of discharging a gas containing free oxygen in at least the proportion contained in air from an inner passage of a flame-producing nozzle; discharging a combustible gas through an annular passage in the nozzle surrounding the inner passage, thus producing a flame having a reducing zone at least towards its perimeter; feeding the irregularly shaped particles to be spheroidized into the inner passage; imparting a swirling motion to particles issuing from the inner pass-age; causing the swirling particles to pass through the flame and its reducing zone thereby melting them at least at their surfaces; and allowing the particles to pass from the flame into a cooling zone.
With the method defined in the previous paragraph an inverse flame is produced.
Preferably, a substantially circular swirling motion is imparted to the particles.
The swirling motion may conveniently be imparted to the particles by imparting a swirling motion to gas issuing from the flame-producing nozzle.
Preferably, a swirling motion is imparted to combustible gas issuing from the annular passage of the nozzle. The surrounding combustible gas transmits its swirling motion to the oxygen containing gas and the particles to be spheroidized which issue from the inner passage of the nozzle.
An inverse flame as described above is normally of the diffusion controlled type which tends to have an inner hollow zone in which no combustion occurs, the oxidizing zone of the flame surrounding the inner zone. As a result of their swirling motion, the particles to be spheroidized are thrown outwardly by centrifugal force from the cold inner zone into a hot zone of the flame.
It will be appreciated that the degree of swirl of particles should be regulated so as to be suificient to maintain them in the hot zone of the flame for an adaquate period of time. In the case of a downwardly directed flame such as is preferably employed, some of the particles might miss the hot zone of the flame by falling more or less vertically downwards, if the degree of particle swirl is too low. On the other hand, if the degree of particle swirl is too great, some of the particles will pass too rapidly through the hot zone of the flame.
It has been found that suitable swirling of the particles to be spheroidized not only improves the efllciency of operation, but also produces improved spheroidizing of the particles.
The amount of swirl imparted to gases and particles issuing from the inverse flarne-producing nozzle can be adjusted to some extent by spacing the mouth of the inner passage from the nozzle mouth and varying the distance between the mouth of the flame-producing nozzle and the mouth of the inner passage.
Preferably, the oxygen containing gas issuing from the inner passage and the combustible gas issuing from the surrounding annular passage of the nozzle have closely similar exit velocities. This assists in maintaining a stable flame since it minimizes the formation of eddies at the interface of the two gases.
It has been found that the spheroidizing operation and also the quality of the spheroidized product in the case of oxidizable particulate initial material, such as ferro-silicon particles, is improved by setting up an additional envelope of reducing gas around the inverse flame.
The additional envelope of reducing gas should be dis tributed very evenly all around the flame to ensure that the hot zone of the flame proper is completely surrounded by an essentially reducing zone of lower temperature. The additional reducing gas should preferably issue from the nozzle at a high velocity. This materially assists in preventing fine particles of material undergoing spheroidizing from escaping from the flame too quickly.
No substantial swirl need be imparted to the additional envelope of reducing gas, although it may be done if necessary.
According to another aspect of the invention a flameproducing nozzle for spheroidizing irregularly shaped particles includes an inner passage for discharging oxygen containing gas and irregularly shaped particles; an annular passage for discharging combustible material, the annular passage surrounding the inner passage; and a tangential inlet into the annular passage.
The flame-producing nozzle may include a supply passage for oxygen containing gas, communicating with the inner passage; and a particle feed passage located within the supply passage for oxygen containing gas and having an outlet directed towards the interior of the inner passage.
The supply passage for oxygen containing gas may be connected to the inner passage through a venturi tube, the outlet of the particle feed passage being located at or near the restricted zone of the venturi tube.
The outlet from the inner passage may be spaced from the mouth of the nozzle.
The flame-producing nozzle may include an additional annular passage for discharging reducing gas, the additional annular pasage surrounding the annular passage for combustible gas.
A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:
FIGURE 1 is a sectional view of a downwardly directed flame-producing nozzle according to the invention.
FIGURE 2 is a section on line IIII in FIGURE 1.
FIGURE 3 is a section on line IIIIII in FIGURE 1.
FIGURE 4 is a section on line IVIV in FIGURE 1.
Irregularly shaped particles of ferro-silicon alloy to be spheroidized are fed through feed hopper 1 into particle feed pipe 2, the outlet 3 of which is directed towards the interior of inner discharge tube 4. Pre-heated air is introduced through inlet pipe 5 into supply tube 6 which surrounds particle feed pipe 2 and which communicates with inner discharge tube 4 through venturi tube 7.
As can be seen from the drawing, outlet 3 of particle feed pipe 2 is located near the restricted zone 7a of venturi tube 7. With this arrangement, flow of air through venturi tube 7 induces a suction effect in particle feed pipe 2, the suction eifect assisting in introducing irregularly shaped particles into inner discharge tube 4. Usually, only a very small quantity of atmospheric air is inducted into particle feed pipe 2 through hopper 1, as under normal conditions feed pipe 2 carries a full load of irregularly shaped particles to be spheroidized.
Air is introduced from inlet 5 into supply tube 6 through vertical apertures 8 so that the air flows straight down supply tube 6 around particle feed pipe 2 without any substantial swirling action. It will be appreciated that irregularly shaped particles entering venturi tube 7 from particle feed pipe 2 are dispersed in air entering venturi tube 7 from supply tube 6. A mixture of pre heated air and irregularly shaped particles is discharged from outlet 9 of inner tube 4. Outlet 9 is spaced from nozzle mouth 10.
Coke oven gas or other suitable combustible fuel gas is introduced tangentially by means of inlet pipe 11 into chamber 12 which is located concentrically around inner discharge tube 4 and converges towards nozzle mouth 10. The tangential introduction of the fuel gas causes it to pass down chamber 12 around inner tube 4 with a pronounced swirling motion to induce at or near nozzle mouth a swirl in the air stream issuing from central tube 4 as well as in irregularly shaped particles dispersed in the air stream.
The air and the fuel gas issuing from nozzle mouth 10 produces a pencil shaped, downwardly directed inverse flame 13 with an oxidizing zone 14 in which the highest temperature in the flame occurs, and a surrounding reducing zone 15 at least towards the periphery of flame 13.
Oxidizing zone 14 is located around a cold air-containing inner zone 20 in which no combustion occurs. Irregularly shaped particles issue from nozzle mouth 10 into cold inner zone 20 and as a result of their swirling motion, the particles are thrown out of inner zone 20 into the hot oxidizing zone 14 where they are melted at least at their surfaces before passing through reducing zone 15 and out of the flame.
Since the particles pass through reducing zone 15 upon leaving flame 13, too extensive an oxidation of the particles is prevented.
It may happen that the cold inner zone 20 extends down to the bottom of flame 13. If the degree of particle swirl is too low, some of the particles might miss the hot zone 14 of the flame by falling more or less vertically downwards. On the other hand, if the degree of particle swirl is too great, some of the particles will pass too rapidly through hot zone 14 of the flame. The degree of swirl should be sufficient to maintain the particles in hot zone :14 of the flame for an adequate period of time to permit them to be melted at least at their surfaces.
The nozzle is so shaped and the air and the fuel gas introduced into the nozzle at such pressures that the air and the fuel gas are discharged at substantially similar exit velocities. This assists in maintaining a stable flame since it minimizes the formation of eddies at the interface between the air and the fuel gas. The exit velocities can be adjusted within limits by raising and lowering outlet 9 of inner discharge tube 4 in relation to nozzle mouth 10. This can be effected by adjusting the position of upper nozzle portion A relative to lower nozzle portion B by adjusting the extent to which upper portion A is screwed into lower portion B at screw-threaded engagement 16.
The degree of particle swirl will depend on the exit velocity of combustible gas issuing from annular chamber 12. The degree of swirl can be adjusted to some extent by adjustment of the position of outlet 9 of inner discharge tube 4 in relation to nozzle mouth 10.
The nozzle also includes annular chamber 17 in communication with outer annular discharge passage 18. A reducing gas, such as coke oven gas, introduced into chamber 17 through inlet 19, is discharged at a high velocity through outer discharge passage '18 to form an additional envelope 15a of reducing gas which completely envelopes flame 13. The reducing envelope 15a is at a lower temperature than flame 13. The high velocity of reducing envelope 15a helps to prevent the finer particles of material undergoing spheroidizing from escaping too quickly from the flame.
As can be seen from the drawings, particle feed pipe 2, supply tube 6, venturi tube 7, inner discharge tube 4, annular chamber 12 and outer annular passage 18 are all located coaxially.
Lower nozzle portion B is cooled by means of water introduced through pipe 21 into cooling jacket 22. Further cooling jackets may be provided if necessary.
Upon passing through flame 13, the particles to be spheroidized are melted at least at their surfaces and assume spheroidal shapes.
After passing out of flame 13, the particles are allowed to cool and solidify. As shown in FIGURE 1, the flameproducing nozzle is mounted on the upper end of cooling chamber 23 and is arranged to direct flame 13 downwardly into chamber 23, which provides a cooling zone. Annular inlet 24 is provided in the top of chamber 23 for directing a curtain of cooling medium down the inner periphery 25 of chamber 23. Cooling medium may also be introduced tangentially into chamber 23 at one or more levels along the height of chamber 23 through one or more peripheral inlets (not shown).
Solidified spheroidized particles may be discharged from chamber 2 3 into a suitable receptacle (not shown). Further cooling means, such as, for example, a heat exchanger, may be provided.
Wet or dry cooling and collection or separation of spheroidized particles may be used as described fully in our -U.S. Patent No. 3,015,852, in which the cooling chamber is referred to as a shaft furnace because of the flame therein.
Cooling chamber 23 is advantageously provided with automatic pressure control means (not shown). This is of particular importance in cases Where the proper dispersion of the particles to be spheroidized in the air stream is dependent on the maintenance of a certain amount of suction on the particle feed pipe 2.
The spheroidized particles produced in accordance with the present invention are characterized by a particular regularity of shape and a great smoothness of surface, substantially Without angular corners.
The process is particularly suitable for the production of particles having a size distribution range below 200 mesh. Alloy particles and particularly ferro-silicon particles of a particle size range below 250 mesh, preferably below 270 mesh, can be spheroidized with good results.
It will be appreciated that many variations in detail are possible without departing from the scope of the invention as defined in the appended claims.
Instead of air, any suitable gas containing free oxygen in at least the proportion contained in air may 'be discharged from inner passage 4. A small proportion of combustible gas, such as coke oven gas, producer gas, or water gas, may be mixed with the oxygen containing gas to improve the flame characteristics. Any suitable combustible fuel gas other than coke oven gas may be discharged from annular passage 12. Similarly, any suitable reducing gas other than coke oven gas may be discharged from outer annular passage 18.
Also, any suitable cooling chamber or other suitable cooling arrangement may be provided.
1. In a method of spheroidizing irregularly shaped particles in which a gas containing free oxygen in at least the proportion contained in air is discharged through an inner passage of a flame-producing nozzle into contact with a combustible gas introduced through an annular passage in the nozzle surrounding the inner passage to thus produce a flame having an inner oxidizing zone and a reducing zone at least towards its perimeter; the irregularly shaped partieles to be spheroidized being fed into the inner passage and axially therealong, the improvement comprising discharging the combustible gas tangentially into said annular passage along a helical path around said inner passage to entrain the particles issuing from the inner passage and cause said particles to undergo a swirling motion as they leave said passage to pass through the flame and its reducing zone thereby melting the particles at least at their surfaces; the oxygencontaining gas being discharged from the inner passage at an exit velocity substantially the same as the exit velocity of the combustible gas issuing from the surrounding annular passage; and cooling the particles as they pass from the flame.
2. A method as claimed in claim 1, comprising forming an additional envelope of reducing gas around the flame.
References Cited by the Examiner UNITED STATES PATENTS 2,451,546 10/ 1948 Forton .5 2,530,345 11/1950 Watts 26412 2,675,295 4/ 1954 Daniels 75-26 3,015,852 1/1962 Hoffman et al. -.5 3,041,672 7/1962 Lyle 264-10 3,059,860 10/1962 Hohn 239423 3,062,638 11/ 1962 Culbertson et al 75-.5 3,093,315 6/ 1963 Tachiki et al 239424 OTHER REFERENCES Article in Journal of Metals, January 1959, pp. 40-42. Tyler, Plasma for Extractive Metallurgy, Journal of Metals, January 1961, pp. 51-54.
DAVID L. RECK, Primary Examiner.
HYLAND BIZOT, N. F. MARKVA, Assistant Examiners.