FI87053B - Process and apparatus for atomizing liquids, expediently melts - Google Patents

Abstract

The invention relates to a process and means for atomizing liquids, for example molten metals, such that the drawn-off liquid flow 6 is subjected to one or more jets 9 of a medium, for example of gas and/or liquid. The acquired mixed flow is conducted towards an obstacle 10 so as effectively to disperse the liquid in the medium, i.e. in order to achieve atomization. <IMAGE>

Description

87053

Method and apparatus for the preparation of liquids, suitably melts For the preparation of liquids, suitably melts

The present invention relates to a process for forming a mist from liquids, preferably metal melts, in which a liquid, preferably a metal melt, is mixed with a jet of gas and / or liquid so that the liquid breaks up into small particles, i.e. a mist. The invention also relates to a means by which this method can be implemented.

This spraying or dusting is carried out by dispersing preferably a vertically lowered stream or other liquid source, preferably by means of horizontal or vertical media streams consisting of gas or liquid.

When liquids are sprayed in such a way that the liquid is decomposed by means of a gas or a fluid, very small particles in certain size ranges are obtained, these size ranges sometimes being very wide. These known methods can be used for most types of liquids. However, they are mainly suitable for producing a powder from a metal melt using a gas, for example nitrogen or argon, as a spray medium. Powders prepared in this way are often referred to as inertly prepared powders and are characterized by a low oxygen content and a spherical shape of the particles.

Powder metallurgical processes using inertly prepared powder involve a number of problems due to the size and / or distribution of the powder particles. The finer and / or narrower fractions of the inertly prepared powder are now desirable in many applications. Such a powder is traditionally obtained by removing the coarse fraction by screening 2,87053, resulting in poor yield, or by using spraying processes using extreme gas flows and pressures. This powder is used only to a limited extent due to its high cost.

When metal melts are sprayed so that the calculated current meets one or more gas jets, instability develops on the surface of the melt, at the interface between the melt and the gas, causing the melt to stretch into thin films. Once these films have reached a certain thickness, they break into wire-like pieces as a result of the surface tension of the melt and then this pieces break into numerous pieces which tend to have a shape with the lowest possible surface energy, i.e. a spherical shape. These spherical droplets solidify very quickly into powder particles as a result of heat being transferred to the gas by radiation and conduction.

Numerous parameters affect the size of the particles formed in a given unit of volume of the spraying process. The surface tension of the melt as well as the density and velocity of the atomizing medium are the parameters that have the most significant effect along with the geometry of the spraying process.

Influencing the surface tension or density is difficult when using a particular melt, spray nozzle, and spray medium, and thus the particle size can be most simply affected by the speed of the spray medium. For this reason, the most well-established spraying processes aim for high speeds by using high pressure in the spray medium and, in the case of gaseous media, by a Laval-structured nozzle. However, the velocity of the gaseous atomizing media decreases very rapidly after the nozzle, so usually only a small part of the atomization process takes place in the range of maximum velocity.

A greater or lesser portion of the melt disintegrates into an area 3 87053 farther from the nozzle, where the velocity is significantly less than the maximum velocity, in some cases as low as 10% of the maximum velocity. This gives a coarse powder with a very wide particle size distribution between the largest and smallest particles.

Another problem is how to make the atomizing medium "stick1" to the liquid, which is why a large amount goes outside the actual spraying area, resulting in poor efficiency.

The object of the method according to the invention is to solve the above-mentioned and other related problems, and it is characterized in that in the vicinity of the medium-blowing nozzle, where the medium jet velocity is still high, a barrier barrier formed by the opposite medium flow is directed against the liquid. larger, which is an advantage for the spraying process, and thereby for efficient mixing of the liquid with the medium, as a result of which the liquid is caused to decompose into small particles, i.e. the spraying is intensified and a fine powder is obtained. Thus, a substantially increased turbulence, which is advantageous for the spraying process, is obtained, whereby the liquid is thus effectively dispersed in the medium.

Efficient spraying is thus achieved by considerably increasing the contact surface between the melt and the spray medium, while providing strong turbulence in the contact area, which is advantageous from the point of view of dispersion and spraying.

In addition, the atomization process takes place only a short distance from the nozzle, where the velocity of the atomizing medium is still high, whereby a large part of the gas participates in the atomization process. In this way, good efficiency is achieved.

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The present method thus makes it possible to radically reduce the average particle size and reduce the size distribution at low cost.

According to the invention, said barrier can be formed by a countercurrent flow of medium gas or fluid, the barrier being formed by a boundary / contact surface between the mixed stream and the countercurrent medium jet.

The method can be applied to both vertical and horizontal spraying processes. When the barrier is chosen appropriately, the method can even form a mist from steel melt or alloys with an even higher melting point.

The invention also relates to a device for carrying out this method, and the characteristic features of this device are defined in the appended claims.

The medium of the medium stream or the opposite stream of material may be water, another liquid such as a liquid gas, or only a gas such as nitrogen or argon or a mixture thereof. Alternatively, the blown gas can be made to rotate.

The method and apparatus of the present invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 shows a means for carrying out the method of the invention, Figure 2a shows an actual spraying process using a gas barrier, Figure 2b shows an example of a barrier nozzle, Figure 3 Fig. 4 shows an alternative means for carrying out the method, Fig. 5a shows a top view of a corresponding spraying process using a gas barrier, Fig. 5b shows this process from the side, and details a barrier producing a barrier, and Fig. 6 shows an alternative spraying process.

Figure 1 shows a vertical spray chamber 1 comprising a casting box 2 for a metal melt. The medium (gas and / or fluid) is introduced through the gas cooler 3 and the compressor 4 to the nozzles in the chamber 1. The powder obtained by spraying passes from the chamber 1 along the piping system to the cyclone 5 to process and separate the powder. A metal melt, for example steel, is discharged from a casting box 2 (Fig. 2a) through a feed arrangement at the bottom of this box as a circular downstream feed stream 6, preferably as circular as possible, into an inert gas-filled spray chamber 1. At the top of the chamber, consists of an annular nozzle or numerous small nozzles. This nozzle forms (these nozzles) an annular gas curtain 9 around the conduit stream, which meets (8) the conduit stream at an acute angle, some distance from the nozzle (s) 7. When the gas encounters the conduit stream, it disintegrates and travels with the gas stream. The barrier 10 according to the invention is located below the point of intersection, at a suitable distance therefrom.

The barrier 10 is preferably a gas barrier 11. It is obtained by directing the gas and / or fluid jet upwards, preferably concentrically with the downcomer and the gas curtain, at a suitable distance from the nozzles, below them. In other words, the second jet is preferably directed immediately towards the first jet 9-6, which first jet contains parts of the melt in its central part 13.

When the two jets meet, the velocity decreases in the collision area, increasing the pressure. As the pressure increases, the gas expands radially outward so that the velocity increases again. If the kinetic energy of both jets 6 87053 is equal, then the resulting direction is substantially radial, i.e., a direction perpendicular to the direction of the jets. The melt in the central part 13 of the first jet changes its direction in the collision area and travels with the radially expanding gas, whereby efficient fumigation is achieved.

The spraying process is further improved if the kinetic energy of the reverse jet is selected to be less or greater than the kinetic energy of the first jet. In this case, the path of the expanding gas is curved, most reminiscent of parabolism (Fig. 2a). The better spraying is due to the entrained parts of the melt that are forced to change direction continuously, making them more efficiently exposed to the gas.

The kinetic energy of the reverse gas stream is preferably selected to be lower than the kinetic energy of the first stream, whereby the phenomenon described above is achieved while the overall direction of the gas / particle mixture is obliquely downwards. If the kinetic energy ratio is reversed, then the overall direction is obliquely upward.

The kinetic energy of the reverse jet may be 10-1000%, preferably 30-60%, of the kinetic energy of the first jet. According to this embodiment, the barrier can be obtained from the nozzle of Figure 2b, having one or more central nozzles 14 for barrier jets. In addition to these, additional nozzles 15 can be used to prevent the liquid (melt) from coming into contact with the undesired parts of the barrier nozzle.

The geometry of the barrier is preferably consistent with the cross-section of the part mixed with the melt 13 of the gas jet. The size of the barrier is suitably such that its longitudinal dimensions are equal to the cross section of the part of the gas stream mixed with the melt at their point of intersection, or even 7 87053 20 times larger, preferably 4-10 times larger than said cross section.

In the method and means described above, in which gas flows out of the nozzles or over the edge of the surface, secondary currents (turbulence) occur at the boundary between the flowing and stationary gas. When forming mist from liquids with a high melting point, this turbulence can cause the molten particles to retract into the nozzles and weld rapidly to the nozzles as well as to other unwanted surfaces. In order to avoid such phenomena on the surface of the nozzles acting as gas barriers, they can be provided with suitably positioned additional nozzles, the function of which is to prevent turbulence in critical areas, thus preventing the adhesion of molten particles. These additional nozzles may be in appearance in accordance with the nozzles shown at 15 in Figure 2b or at 18 in Figure 3.

Figure 4 shows a horizontal spraying apparatus comprising a spraying chamber 19 and a cyclone 20. This spraying apparatus comprises a closed system which is preferably maintained at a certain overpressure (see Figures 1 and 4). This overpressure can correspond, for example, to a 500 mm water column, in which case no air can enter the system. As mentioned above, the casting box 2 is located at one end of the box (1, 19). Figures 5a and 5b show spraying implemented with the apparatus of Figure 4. The medium 22 flows from the nozzles 21 (for example from an elongate, slit-shaped nozzle or a row of small nozzles) towards the downstream stream 23. The mixture stream thus obtained then encounters an obstacle (fixed obstacle or an obstacle formed by one or more nozzles 25), whereby its direction changes, whereby excellent atomization is achieved. In Fig. 5b, the additional nozzles are arranged in one slot-shaped nozzle 26 and in several small separate nozzles 27. The nozzle 26 can even form a barrier itself.

In Figures 5a-b, in order to obtain the mixture jet 24, the flow phenomenon which occurs when two gas or fluid streams meet at a certain angle is utilized.

It is known that at or immediately before the intersection of two jets of medium meeting at a certain angle, there is a flow phenomenon which controls the process more or less depending on the magnitude of said angle. When the angle is small, i.e. less than 5 ', the dominant feature is the injection effect due to the vacuum immediately before the point of intersection, while at larger angles, e.g. 120 *, there is a backward flow of medium with respect to the main flow direction of the medium jets.

Both of these phenomena can be exploited by selecting the angle between the two media jets 22, 22 so that there is a backward flow of the medium and at a short distance the injection effect draws it back into the medium jet. The result is that a zone is formed in front of the intersection point, in which there is no predetermined direction, but in which there are two vortices and a continuous exchange between the reversible medium and the retractable medium. Changing the angle increases or decreases this zone. The angle between the jets of medium may be 0-60 ", but is preferably 5-20 *.

The nozzles 21, 22 can be arranged in such a way that in the vertical apparatus two parallel jets of horizontal medium are obtained, the vertical dimension of which is large in relation to the width and which form an angle with respect to each other in the horizontal plane. In this case, the zone described above is formed. The conducted stream 23 flows from top to bottom in a vertical zone formed along the entire height of the nozzle. This stream gradually disintegrates as it travels downward and mixes with the urinating nebulizing medium.

Media jets with a considerable dimension in one direction can be provided, for example, by slot-shaped nozzles or by a plurality of circular nozzles arranged in a row close to each other. Depending on the prevailing pressure and the medium used, the nozzles for the medium jets can be designed for conditions where there is a negative pressure or a supercritical pressure (Laval nozzle).

When the melt flow is adjusted correctly to match the capacity of the medium nozzle, mixing, i.e. partial spraying, takes place over the entire height of the nozzle.

The advantage of the nozzles 21 arranged as described above is that they can achieve a more homogeneous mixing of the liquid with the medium (partial atomization), as a result of which a narrower particle size distribution is obtained even after passing the barrier. Such an arrangement of the nozzles 21 can also be used for complete spraying without obstruction, whereby particles with a narrow size distribution but a larger average particle size can be produced.

Figure 6 shows an alternative embodiment of the method and device according to the invention. An electric arc 30 is formed between the electrodes 28, 29. The medium flows 31 (gas and / or 'fluid') are directed towards this electric arc and the medium flows 32 from the opposite direction act as a barrier. In this way, efficient atomization of the liquid 35 formed in the electric arc is achieved.

In this case, the liquid to be slid is obtained from at least one electrode 29. However, the liquid can also be obtained from a solid body which is melted by a laser or the like (not shown). In Fig. 6, the feeding of the electrodes or the laser can be realized by the feeding device 34. The nozzles of both the first medium and the barrier medium can be annular or they can consist of several small nozzles. The method according to Figure 6 is preferably carried out in a chamber as previously described (not shown).

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The particles formed by spraying are drawn into the gas jets towards the other end of the chamber and solidify into a powder before reaching the end of the chamber because heat is transferred to the gas by radiation and conduction. The chamber comprises, preferably at one of its ends, an outlet towards which the gas / powder mixture flows.

The outlet of the chamber is connected by pipes to a cyclone where the powder and gas are separated. After separation, the gas can be led through a gas cooler to a compressor to recirculate the gas back to the atomizing nozzles. The system comprises other necessary components, such as valves, cooling devices and control devices for controlling the gas pressure, temperature and various medium flows, etc.

The means and methods described above can be modified in many ways within the scope of the claims.

Claims (11)

11 8 7053 A method for spraying liquids, preferably metal melts, by mixing this liquid, preferably a metal melt, with a gas and / or liquid medium jet (6, 22, 31) so that it disintegrates into small particles, i.e. it is obtained as a mist, characterized in that the medium-blowing in the vicinity of the nozzle, where the medium jet velocity is still high, a barrier barrier formed by the opposite medium flow is directed against the medium to multiply the liquid / melt-medium contact surface, which is advantageous for the spraying process, and to effect that is, the spraying is intensified and a fine powder is obtained. A method according to claim 1, characterized in that the barrier (10, 25, 32) is formed by a gas and / or fluid stream whose direction is substantially opposite to the direction of the jet of liquid and medium and which is directed towards this jet. . Method according to Claim 1 or 2, characterized in that a barrier (11) is used in the method. . ·. the geometry of which is preferably consistent with the cross-section of the liquid-containing part (mixed jet) of the medium jet at the point where it meets the obstacle, and that longitudinal dimensions equal to or not more than 20 times the said cross-section are determined for the obstacle, preferably 4-10 times larger than said pouch. Method according to one or more of the preceding claims, characterized in that the barrier gas and / or liquid stream is produced by one or more nozzles and in that the kinetic energy of the stream obtained therefrom is 10-1000%, preferably 30-60%, of the medium jet. 2 87053 (s) of kinetic energy. Method according to one or more of the preceding claims, characterized in that additional nozzles (15, 18, 26, 27) are provided in the gas and / or liquid nozzles, which prevent the liquid / melt from coming into contact with the gas nozzles or the obstruction body by means of a turbulence / injection effect. with unwanted parts. Method according to one or more of the preceding claims, characterized in that the liquid is obtained by transferring energy to the metal or alloy (28) and that the medium (31) is introduced into this liquid (35) to obtain a mixture jet directed at the mixture, e.g. towards the fluid jet (32) flowing in the opposite direction. Method according to Claim 6, characterized in that the energy is produced by means of an electric arc, a laser or the like. Means for carrying out the method according to one or more of the preceding claims, which means comprises a casting box (2) for a liquid, for example a metal melt, and nozzles (7, 21) for medium jets adapted to meet from the ore box or. . a current (6, 23) derived from another liquid source (35), characterized in that said means also comprises an obstacle in the form of a counter-jet and located in the path of the liquid-medium mixture jet and intended to significantly increase the liquid / melt and medium jet to effectively disperse the liquid in the medium and thus enhance the spraying of the liquid. 87053 Device according to Claim 8, characterized in that one or more nozzles (12) are provided for forming the barrier, which are preferably directed in the opposite direction to the mixture flow and which have the function of directing one or more gas or liquid jets towards the mixture stream to increase spraying. Device according to claim 8, characterized in that the barrier-forming arrangement also comprises additional nozzles (15, 18, 26, 27) which prevent the liquid from coming into contact with the gas nozzles and / or the undesired parts of the barrier body due to turbulence and / or due to the injection effect. Means for carrying out the method according to one or more of claims 1 to 7, characterized in that the metal or alloy is adapted to be conducted as a wire, rod or powder (28, 29), and that energy is supplied to this metal or alloy by a laser or electric arc (30). ) or the like to melt it, after which the jets (31) of the liquid (35) thus obtained are subjected and the mixture stream thus obtained is directed against an obstacle, for example the opposite jet of medium (32), in order to effectively spray the liquid. ,, 87053