JPH0459363B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method and apparatus for producing metal powder by an atomization method. In particular, the present invention relates to a method and apparatus for preventing blocking caused by backflow pressure within a spray cone during the manufacturing process. (Prior Art) One of the methods for producing metal powder is an atomization method. This method is a method in which molten metal is pulverized by an injected atomizing medium. That is, the atomizing medium is injected as a spray jet from an atomizing nozzle,
Molten metal is injected into the nozzle and pulverized. (Problems to be Solved by the Invention) However, in the production of metal powder, blocking tends to occur. Blocking is a phenomenon in which the molten metal injected into the atomizing nozzle solidifies within the atomizing nozzle without being atomized, thereby blocking the space within the nozzle. When blocking occurs, atomization work is interrupted. When this is severe, the atomizing nozzle is covered with molten metal, so in severe cases the atomizing nozzle may become unusable. As described above, blocking is a major problem in the production of metal powder by atomization because it obstructs operations and disrupts the production process. There are three main causes of blocking. One of these is disturbances such as twisting and scattering of the molten metal falling into the atomizing nozzle. Disturbance of the focus of the spray jet of the atomizing nozzle also poses a problem. The most important thing is the spray conditions of the atomizing nozzle. That is, the greater the pressure P of the spray medium, the flow rate Q, and the apex angle β of the spray cone, the greater the risk of causing blocking. The quality of the powder produced is improved by increasing the values of the parameters that determine these spray conditions. That is, the average particle size of the powder becomes smaller and the apparent density of the particles also decreases. In other words, if the spray conditions are set to improve powder quality, blocking is likely to occur. As a means to solve such problems,
For example, Japanese Patent Application Laid-Open No. 114467/1983 proposes attaching a suction pipe below an atomizing nozzle to increase the flow rate of suction air to the nozzle. However, it is impossible to freely control the amount of suction only by installing such a suction pipe, and there is also a limit to the increase in the suction force. Therefore, the range of spray conditions that can be selected is also quite limited. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method and apparatus for producing metal powder by atomization, which allows a wide range of atomization conditions to be freely selected and which does not cause blocking. (Means and effects for solving the problem) In order to achieve this objective, the present inventor observed the actual atomization situation, set the actual atomization conditions, and set the atomization nozzle while spouting the atomization medium. Perform hydrodynamic measurements on the inside,
We investigated the process leading to blocking and its causes. The following description will be made regarding atomization using a liquid spray medium such as water or oil. However, it goes without saying that the present invention can be similarly applied to other types of atomizing nozzles such as gas atomizing nozzles. In the early stages of blocking, a phenomenon can be observed in which the molten metal that has not been atomized solidifies inside the atomizing nozzle, becomes small pieces, and repeatedly adheres to and falls off. The particles then grow and block the space within the nozzle, eventually leading to seizure and finally blocking. This phenomenon occurs when the molten metal injected into the atomizing nozzle is splashed up by the spray jet. In order to investigate the cause of this, the backflow pressure Pr in the atomizing nozzle was measured using a backflow pressure probe while the spray medium was being ejected under actual spraying conditions.
As a result, an area where the airflow was flowing backwards was observed near the top of the spray cone. Here, the term "spray cone" refers to an inverted conical surface forming the upper surface of the spray jet ejected from the slit of the atomizing nozzle. A backflow pressure probe is a tubular pressure measurement instrument similar to a pitot tube, and the pressure in the space above the atomizing nozzle is used as a reference, and the pressure in the pore opened below the tip of the probe is compared to the above reference pressure. Measured as differential pressure. If there is airflow from above to below at the measurement part (tip) of the probe, it will indicate negative pressure (positive flow pressure) compared to the reference pressure, and conversely, if there is airflow from below to above, it will indicate positive pressure (backflow pressure). ). Based on the actual blocking situation and hydrodynamic measurement results described above, blocking is caused by the force of the airflow (backflow region) from below upwards that is generated near the top of the spray cone where the spray jet collides with a part of the molten metal. It was found that the origin of the problem was that the particles were splashed up from the water, adhered to the inner diameter of the atomizing nozzle, and solidified. This backflow region was caused by the collision of the spray medium at a high pressure of approximately 200 kg/cm ² .
Therefore, the greater the pressure P, flow rate Q, and spray cone apex angle β of the injected spray medium, the greater the collision force, the greater the strength of the backflow, and the more likely blocking will occur. Therefore, it can be seen that blocking can be prevented by suppressing the backflow region.
As a result of further experiments and research, it was found that there is a close relationship between the occurrence of this backflow region and a decrease in the flow rate Qg of the airflow drawn into the atomizing nozzle from above. Therefore, the inventor of the present invention came up with the idea that the backflow region can be suppressed by ensuring the suction flow rate Qg. This point will be explained in more detail. In a series of experiments, it was found that as the apex angle β of the spray cone was increased while the pressure and flow rate of the spray medium were kept constant, the suction flow rate Qg gradually decreased. Therefore, as β becomes larger, the backflow region becomes stronger and blocking becomes more likely to occur. Therefore, even when β is increased, the suction flow rate Q
If g can be ensured, the backflow region can be suppressed, and atomization can be performed without causing blocking. Therefore, in order to prevent blocking, prior to atomization, it is necessary to operate the atomization nozzle under spray conditions equal to those of actual atomization, and measure the backflow pressure with a backflow pressure probe while changing the suction air flow rate. Accordingly, the suction flow rate Qg that eliminates the backflow region may be determined, and then atomization may be performed while ensuring the determined suction flow rate. Here, the backflow pressure Pr is 0 or negative (Pr≦0) inside the atomizing nozzle inner diameter and spray cone.
When , it is judged that the backflow area has disappeared. Alternatively, the suction flow rate may be varied under various spraying conditions, the backflow pressure may be measured in advance for each case, and a suitable empirical formula may be created to determine the conditions for eliminating the backflow region and the operation may be conducted in accordance with this formula. By the way, in order to freely control the suction flow rate and ensure a predetermined amount, the following means may be used. A suction nozzle is installed below the atomizing nozzle, and a suction pipe is further attached below the suction nozzle. And airtightness between them is maintained to prevent gas from entering from the outside. In order to further enhance the suction effect, it is preferable that the entire inner cross section of the lower end of the suction tube be filled with the spray jet ejected from the atomizing nozzle and the suction nozzle. In this case, the dimensions of the suction nozzle and the suction tube may be determined so as to satisfy the conditions. With this configuration, the suction nozzle and the suction tube work together as one type of ejector, and a strong suction force can be obtained. The source of the suction power is the jet sprayed from the suction nozzle and the atomizing nozzle. Therefore, whether the jet fluid is liquid or gas, the suction effect can still be obtained. Further, the pressure and flow rate are selected depending on each case, and there are no particular restrictions. Therefore, the feature of the method according to the present invention is that a spray medium is injected as a spray jet from an atomizing nozzle to form a spray cone consisting of the upper surface of the jet in the space within the nozzle, and the spray cone is formed from the upper surface of the spray cone. In a method of producing metal powder by injecting molten metal from above a spray cone formed by: a. the atomizing nozzle necessary for maintaining positive flow pressure inside the spray cone under spray conditions during production of the metal powder; a step of determining the flow rate of suction air into the internal space before starting injection of the molten metal; b. injecting the atomizing medium from the atomizing nozzle according to the atomizing conditions to form a spray jet, and sucking it below the atomizing nozzle; arranging a nozzle, further arranging a suction pipe below the nozzle, suctioning the airflow in the space within the atomizing nozzle by a negative pressure generated by a fluid injected from the suction nozzle downwardly inside the suction pipe; a step of ensuring suction of an air flow equal to or higher than the suction air flow rate determined in step a) into the space within the atomizing nozzle; c. while maintaining the suction air flow rate of step b);
and a step of producing metal powder by injecting molten metal into a spray cone. Further, the device according to the present invention is characterized in that the atomizing medium is injected as a spray jet from an atomizing nozzle to form a spray cone consisting of the upper surface of the jet in the space inside the nozzle, and molten metal is injected from above the spray cone. An apparatus for producing metal powder, comprising: a suction nozzle disposed below an atomizing nozzle for injecting fluid downward; and a suction pipe disposed below the suction nozzle, the atomizing nozzle , the suction nozzle and the suction tube are held airtight with each other, and the airflow in the space within the atomizing nozzle is suctioned by the negative pressure generated thereby. (Example) Next, an example of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic vertical cross-sectional view of the present invention implemented in an oil atomizing device using an annular atomizing nozzle, and FIG. 2 is a partial vertical cross-sectional view showing enlarged details of the main parts thereof. Molten metal 2 supplied into the tundish 1 flows down as a molten metal injection stream 5 from a tundish nozzle 4 that is opened and closed by a stopper 3 during metal powder production. On the other hand, below the tundish 1, an atomizing nozzle 6, a suction nozzle 7, and a suction pipe 8 are arranged in a manner that maintains airtightness with each other, and a support member 9a inserted between the atomizing nozzle chamber 9 and the atomizing tank 10 is provided.
Supported by The atomizing nozzle chamber 9 and the atomizing tank 10 are communicated with each other through a communicating pipe 11. Before the start of metal powder production, inert gas is first introduced from the inert gas supply piping 14 to exhaust the air inside the atomization device and replace it with inert gas. The spray medium (oil) and the suction fluid (oil) are supplied from the slits 16 and 19 downwardly from the spray jets 17, 13, respectively.
18 and a suction jet 20.
The atomizing nozzle chamber 9 and the atomizing tank 10 communicate with each other through a communication pipe 11. Therefore, nozzles 6 and 7
When the operation starts, the gas in the atomizing nozzle chamber 9 is sucked into the nozzle 6 to generate a suction flow rate 21 (see Fig. 2), and a strong ejector action in the nozzles 6, 7 and the suction pipe 8 (this is caused by the jet 17 ,1
8), the upper surface of the jet 20, and the inner circumferential surfaces of the nozzles 6 and 7. It is released into the atomization tank 10 along with the medium. On the other hand, a flow from the atomization tank 10 to the nozzle chamber 9 is generated in the communication pipe 11 . Next, the molten metal 2 is supplied into the tundish 1 until it reaches a predetermined head, and after being allowed to stand still for a while, the stopper 3 is raised and the molten metal is injected into the center of the atomizing nozzle 6 through the tundish 4. The injected stream 5 is pulverized into fine particles by the spray jet 17, which is then removed from heat and solidified into powder, which is discharged from the suction pipe 8 together with the jet 18 and settles at the bottom of the atomization tank 10. The inverted cone forming the upper surface of this jet 17 is the spray cone 1.
7a, the symbol A (FIG. 2) indicates its apex, and the symbol β indicates its apex angle. In the present invention, the suction jet 20 from the suction nozzle 7 does not have a direct effect on pulverization, but only serves to increase the flow rate of the suction airflow 21. Next, a first embodiment of the method according to the present invention using the above-described embodiment apparatus will be described with reference to FIG. 3 and Table 1 below.

[Table] Figure 3 shows the distance Z on the central axis of the atomizing nozzle 6 and the backflow pressure Pr (Kg/Kg/
cm ² ). As already mentioned, the backflow pressure Pr is a value actually measured by a backflow pressure probe, with the pressure directed from the bottom to the top being positive. The atomizing nozzle 6 and the suction nozzle 7 according to the first aspect of the method of the present invention are driven as follows. First, while the suction nozzle 7 is stopped, the atomizing nozzle 6 is driven under almost the spraying conditions used in actual production of metal powder. That is, the oil pressure P and oil amount Q are set to the values during actual manufacturing, and the spray cone apex angle β is set to a value slightly larger (for example, about 4 degrees) than the desired value in actual manufacturing (the reason for this is (This will become clear from the explanation below). Graph a in Figure 3 shows the oil pressure P of the atomizing nozzle 6 as 170Kg/cm ² .
Oil amount Q is 109m ³ /H, spray cone angle β is
Shows the measured value of backflow pressure Pr when set to 48.0°. After that, increase the oil pressure Ps of suction nozzle 7 to 50, 70, 100.
Increase the pressure to Kg/cm ² and measure the backflow pressure Pr. (See graphs b, c, and d. Point A in each graph indicates the position of vertex A of spray cone 17a. Graph e is a reference example.) As shown in FIG. In the case of graph a where 7 is not activated, spray cone 1
There is a backflow pressure (Pr≧0) below the point approximately 18 mm above the apex A of 7a. graph b,
In the case of c, the spray cone 1 is also near point A.
A backflow pressure is generated in the internal region of 7a.
However, in the case of graph d, there is a positive flow pressure (flow pressure directed from above to below) at point A and all points inside the spray cone 17a above it, and if no blocking occurs, the state is suitable for atomization. (The graph e also shows a state suitable for atomization in which the inside of the spray cone 17a is entirely at positive flow pressure.) In this way, the oil pressure Ps of the suction nozzle 7 is expressed as a graph a.
When changing from the state to the state d, the spray jet 17 is sucked downward and the spray cone apex angle β decreases from 48.0 degrees to 44.0 degrees. On the other hand, during this period, the suction air flow rate Qg into the atomizing nozzle 6 is
An increase was observed from 670 m ³ /H in a to 1250 m ³ /H in d (see Table 1). Therefore,
In actual metal powder production, when the spray conditions of the atomizing nozzle 6 are given as d in Table 1, the amount of metal powder Q
It is sufficient to maintain g at approximately 1250 m ³ /H or more. Therefore, under the spray condition d, the predetermined suction air flow rate is 1250
While maintaining m ³ /H, the stopper 3 is raised to open the tundish nozzle 4 and the injection of the molten metal 2 is started. If atomization is continued after that while maintaining a predetermined suction air flow rate, blocking can be prevented from occurring. For reference, the same spraying conditions (P = 170
When atomization was performed without any suction using the device according to the present invention at a temperature of Kg/cm ² , Q = 109 m ³ /H, β = 44.0°, blocking occurred. Next, a second embodiment of the method of the invention will be described using the empirical parameter Bl as an index of blocking. The value of the suction air flow rate Qg required to eliminate the backflow pressure region within the spray cone 17a depends on each device and spray conditions. Therefore, in the method of the first embodiment described above, each time the spray conditions are set, the backflow pressure must be measured to determine the required suction air flow rate. However, in the second implementation method using the parameter Bl, in which measurements are performed while changing the spray conditions and the suction air flow rate to various values in advance, it is not necessary to perform measurements in each case. This is as explained below. Actual measurements as shown in Table 1 were carried out by varying the spray conditions and suction amount to various values, and the parameters given by the following equation were created based on experience as the blocking index. Bl=Qg/[Q√(1-cosβ/2)] Here, Qg is the suction air flow rate, and P, Q, and β are the oil pressure of the spray medium, the oil amount, and the spray cone apex angle. (Table 1 shows examples of the values of Bl in cases a to e.) Furthermore, as a result of the various actual measurements described above, in the device examples shown in FIGS. 1 and 2, the parameter It has been found that when Bl≧15000 holds true, no backflow pressure is generated inside the spray cone 17a (that is, the entire area within the spray cone becomes positive flow pressure). Therefore, the spray conditions P in atomization,
When Q and β are given, actual atomization can be performed while maintaining the suction air flow rate Qg to satisfy Qg≧15000 [Q√(1−cosβ/2)]. Next, Table 2 shows the values of Bl and the presence or absence of blocking when the spray conditions and suction air flow rate were varied.

[Table] Note that the value of the parameter Bl required to eliminate the backflow pressure region differs depending on the specific device, but if this value is determined in advance by actual measurements for each device, it can be used in the same way as in the above case. The amount of suction air required to eliminate blocking can be determined. (Effects of the Invention) In the method of the present invention, the suction air flow rate necessary for eliminating blocking is maintained under each spraying condition selected over a wide range. The device according to the invention also provides a specific means for maintaining this suction air flow rate. Therefore, it is possible to completely eliminate the occurrence of blocking due to backflow pressure even under spray conditions for producing high-quality metal powder, which would normally cause blocking. In fact, we have already been able to increase the spray cone apex angle from 37°, which was considered to be the limit in the past, to 45°.

[Brief explanation of the drawing]

FIG. 1 is a schematic vertical cross-sectional view of an atomization device including an embodiment of the present invention; FIG. 2 is a partial vertical cross-sectional view showing a part of the device shown in FIG. 1 on an enlarged scale; and FIG. It is a graph showing the measurement results of backflow pressure in the method of the present invention. DESCRIPTION OF SYMBOLS 1... Tundate dish, 2... Molten metal, 3... Stopper, 4... Tundate nozzle, 5... Injection flow, 6... Atomize nozzle, 7... Suction nozzle, 8... Suction pipe, 9... atomize nozzle chamber,
9a...Supporting member, 10...Atomization tank, 11
...Communication pipe, 12, 13, 14...Piping, 16...
...Slit, 17...Spray jet, 17a
... Spray cone, 18 ... Spray jet, 19 ... Slit, 20 ... Suction jet.

Claims (1)

[Scope of Claims] 1. A method for producing metal powder by injecting a spray medium from an atomizing nozzle to form a spray jet, and injecting molten metal from above a spray cone formed by the upper surface of the spray jet, comprising: a a step of determining, before starting injection of molten metal, a flow rate of suction air into the atomizing nozzle necessary to maintain a positive flow pressure inside the spray cone under spray conditions during metal powder production; b. A spray medium is injected from the nozzle under the above-mentioned spray conditions to form a spray jet, and a suction nozzle is disposed below the atomizing nozzle, and a suction tube is further disposed below the atomization nozzle, and the inside of the suction tube is drawn from the suction nozzle. A step of suctioning the airflow in the space within the atomizing nozzle using the negative pressure generated by the fluid injected downward, and suctioning an air flow rate greater than the suction air flow rate determined in step a) into the atomization nozzle. A method for producing metal powder, comprising: c) producing metal powder by injecting molten metal into a spray cone while maintaining the suction air flow rate in step b). 2. Determination of the suction air flow rate in step a) is as follows:
The method is experimentally performed by actually measuring the flow pressure inside the spray cone while changing the suction air flow rate while injecting the spray medium from the atomizing nozzle under the spray conditions used during actual manufacturing. A method for producing metal powder according to scope 1. 3. The method for producing metal powder according to claim 1 or 2, wherein the spray conditions include the spray pressure, flow rate, and apex angle of the spray cone of the spray medium. 4. Determination of the suction air flow rate in step a) is as follows:
changing the spray conditions and suction air flow rate in advance, measuring the flow pressure inside the spray cone under each spray condition and suction air flow rate, and adjusting the forward flow inside the spray cone under each spray condition based on the measured values. Determining the suction air flow rate by determining an empirical formula that gives the suction air flow rate necessary to maintain the pressure, and substituting the spraying conditions during actual manufacturing into the empirical formula. A method for producing metal powder according to any one of claims 1 to 3, characterized in that: 5 In an apparatus for producing metal powder by injecting a spray medium from an atomizing nozzle to form a spray jet and injecting molten metal from above the spray cone formed by the upper surface of the spray jet, disposed below the atomizing nozzle. The atomizing nozzle, the suction nozzle, and the suction tube are held airtight with each other, and the atomizing nozzle, the suction nozzle, and the suction tube are held airtight with each other. Metal powder manufacturing equipment. 6. The metal powder manufacturing apparatus according to claim 5, wherein the atomizing medium ejected from the atomizing nozzle and the fluid ejected from the suction nozzle fill the entire internal cross section of the lower part of the suction tube. .