A PROCESS AND PLANT FOR PRODUCING ATOMIZED METAL POWDER, METAL POWDER AND THE USE OF THE METAL POWDER

Abstract: The present invention relates to a process for manufacturing atomized metal powder in an atomization plant comprising a casting box, a reactor vessel, a powder container and sedimentation equipment. The production process takes place with controlled thermal balance. The invention also relates to an atomization plant, atomized metal powder and the use of the metal powder as coolant in the manufacture of steel.

Description: A PROCESS AND PLANT FOR PRODUCING ATOMIZED METAL POWDER. METAL POWDER AND THE USE OF THE METAL POWDER The present invention relates to a process for producing atomized metal powder in an atomization plant comprising a casting box, a reactor vessel, a powder container and sedimentation equipment. The invention also relates to the atomization plant, atomized metal powder produced according to the process and the use of the metal powder. One of the problems in manufacturing atomized metal powder is that the thermal balance in the reactor is not in balance and that critical temperatures occur. This entails increased risk of explosion since the firing temperature and partial pressure are reached in uncontrolled manner. Another problem is that if the pressure of the spray coolant is too high the powder particles will be deformed, becoming uneven and pointed in shape. High temperature of the spray coolant also causes the formation of waves on the surface of the liquid. The object of the present invention is to provide a solution to these problems. According to the invention they are solved by introducing atomizing medium into the reactor vessel via primary nozzles in the upper part of the reactor. Coolant is then supplied at low pressure via at least one secondary supply arrangement in the upper part of the reactor vessel, arranged in combination with the nozzles for atomizing medium. Coolant and atomizing medium are withdrawn from the lower part of the reactor and then recirculated via a number of transport arrangements and sedimentation equipment. Some of the metal powder is removed directly from the reactor, down into a powder container. The rest of the metal powder is separated through sedimentation in sedimentation equipment. The embodiment described above, and other embodiments of the invention, are defined in the dependent claims. Description of the invention: From a casting box a stream of molten metal, preferably steel, flows into the reactor vessel. The stream is disintegrated by atomizing medium flowing under high pressure from primary nozzles in the upper part of the reactor. Secondary coolant is allowed to flow under low pressure from at least one annular extruder in connection with the primary nozzles. The coolant flows down through the gas chamber of the reactor vessel and forms cooling curtains. The gas-filled part of the reactor is therefore smaller than the corresponding gas chamber in conventional atomizing plants. Large quantities of coolant at low pressure achieve efficient cooling of the powder particles without them become deformed. They retain their spherical shape since the thrust with which the coolant encounters the particle surface is limited. The desired final product is thus obtained and at the same time the thermal balance necessary for safety of the process is also achieved. Wave formation is greatly suppressed through the supply of secondary coolant through the annular extruders and the variation in the path of the powder particles from vortex to liquid surface is thus reduced. In order to attain constant conditions in the reactor vessels the coolant balance must be at equilibrium during the atomizing period. The same amount of coolant must be removed from the reactor vessel as is supplied during the same time period. The falling rate of metal powder with a size of lOOμ is in the order of magnitude a few cm/ sec. So that the reactor plant does not become unreasonably large the bottom of the reactor vessel has been provided with an inner cone so that the powder formed is guided down through the bottom outlet and into a powder container, known as a wet container. The coolant is sucked out via a specially shaped suction chamber arranged in the lower part of the reactor vessel. Only marginal quantities of powder particles larger than lOOμ are drawn out through this suction chamber. Particles smaller than lOOμ, preferably smaller than 50μ, are carried out with the coolant. Powder of such small particle size is very attractive for certain purposes and it is therefore important that this fraction can be salvaged in a simple and efficient manner without extra work operations. This can easily be achieved by allowing the coolant withdrawn to sediment in at least two cylindrical sedimentation containers having conical bottoms. The inclination of the cones shall at least exceed the angle of repose of the powder. The sedimentation container is dimensioned with a good margin to hold the coolant and atomizing medium required for one charge of powder in the atomizing process. The height and diameter of the container must be optimized to allow all powder particles larger than 20μ to have time to settle between two charges. The inlet for coolant and atomizing medium into the container shall also be designed and placed to facilitate sedimentation. From the above, therefore, it is evident that at least two sedimentation containers are necessary for the atomizing process. The coolant withdrawn passes a suction pump. Since the sedimentation container holds the coolant and atomizing medium requirement for a full charge, atomization and subsequent cooling of the powder occurs down to solidification temperature with exactly the same cooling and atomizing medium temperature throughout the charge. This results in a powder with optimal reproducibility with regard to atomizing, particle shape and distribution of carbon in the powder produced. The coolant is introduced into a storage tank having an inlet part in the form of a sedimentation basin. The sedimented powder particles, the majority of which are smaller than lOOμ, are collected in a separate wet container. The coolant freed from powder is recirculated to the reactor vessel via a heat exchanger and with the aid of high-pressure pumps through the spray nozzles as atomizing medium and through the annular extruders as secondary coolant, respectively. The part-functions described above cooperate to produce an efficiently operating atomization plant with great flexibility with regard to the properties and shape of the powder produced. A small quantity of the atomizing medium, which preferably consists of acyclic and /or isocyclic hydrocarbon compounds such as paraffin or diesel oils, is carbonized to carbon and hydrogen in the atomizing process. This carbon is completely absorbed by the powder particles, primarily in their outer layer. The hydrogen formed at carbonization increases the pressure in the gas part of the reactor and must therefore be removed. This is achieved via a liquid lock. Detailed description of the invention: The invention will be described in more detail with reference to the accompanying drawings. Figure 1 shows a reactor vessel according to the invention. Figure 2 shows an atomization plant in which the coolant i s recirculated in accordance with the invention. The atomizing part of the atomization plant comprises, besides the reactor vessel 1, a casting box 2 for metal melt to be atomized. A metal stream 3 leaves the casting box 2 and at least one nozzle 4 is directed towards this stream. Atomizing medium leaves the nozzle 4 under sufficiently high pressure for the metal stream 3 to be atomized. Large quantities of secondary coolant leave supply arrangements 5 which may be annular extruders, at low pressure. A curtain 6 of coolant is formed which cools the metal powder and causes it to solidify into preferably spherical particles. A liquid lock 7 is arranged in the reactor wall to evacuate the overpressure formed when the atomizing medium is carbonized. The bottom 8 of the reactor vessel is conical so that powder particles larger than lOOμ will bedeposited and carried out to a powder container 9, not shown in Figure 1. To prevent disturbance of the liquid balance, coolant is withdrawn through suction means 10. Finer powder particles, the majority of which are smaller than lOOμ, accompany the coolant out of the reactor vessel. Fine powder and coolant are pumped by a low-pressure pump 11, see Figure 2. Coolant containing fine powder is carried to a sedimentation container 12 which is large enough to hold coolant and atomizing medium for a whole charge. A low-pressure pump 13 pumps coolant and atomizing medium, freed from particles by means of sedimentation, back to the reactor vessel 1 via a heat exchanger 14. A small quantity of the medium is pumped out via the atomizing nozzles 4 by a high-pressure pump 15, in jets directed towards the metal stream 3, thus atomizing said metal stream. Most of the medium is supplied under low pressure through the annular extruders 5, and cools the metal powder formed. The metal powder formed is spherical in shape and preferably consists of steel. The surface layer of the powder particles has increased carbide- bound carbon as a result of the present atomizing process. The size distribution of the particles is >150μ, 150-20μ and <20μ, preferably >100μ, 100-20μ and <20μ. The powder particles, also known as IPS powder, are extremely hard because of the high proportion of carbide- bound carbon in the surface layer. The hardness of the IPS powder is approximately 900 as compared with metal powder from conventional atomizing processes where the hardness is approximately 200. Thanks to its hardness, high carbon content and low oxygen content, the IPS powder can be used with tool-polishing effect. The IPS powder with a particle diameter of less than lOOμ can therefore be used for pressure die casting up to a content of approximately 10%.

Claims: CLAIMS 1. A process for producing atomized metal powder in an atomization plant comprising a reactor vessel (1), a casting box (2), a powder container (9) and sedimentation equipment (12), characterized in that atomization occurs in said reactor vessel (1) by means of an atomizing medium being introduced through one or more primary nozzles (4), and in that a part of the atomizing medium supplied is carbonized to carbon and hydrogen in the gas part of the reactor vessel, after which coolant is supplied at low pressure via at least one secondary supply arrangement (5) in the upper part of the reactor vessel in such a manner that the coolant flows down through the gas chamber of the reactor vessel, from whence the powder particles formed at atomization are carried down into the powder container (9) by the bottom of the reactor vessel being in the shape of a cone, and the coolant is carried out together with finer powder particles due to the suction action from suction means (10) in the lower part of the reactor and pumped to said sedimentation equipment (12) where accompanying powder particles are separated out and from whence coolant is recirculated to the reactor vessel (1). 2. A process as claimed in claim 1, characterized in that said supply arrangement (5) comprises an annular extruder. 3. A process as claimed in claim 1 or claim 2 , characterized in that large quantities of coolant at low pressure cool the powder particles and that the sedimentation equipment (12) is dimensioned to contain the entire quantity of coolant required to cool a full charge of powder. 4. A process as claimed in any of claims 1-3, characterized in that the carbon carbonized from the atomizing medium, which preferably consists of acyclic and/ or isocyclic hydrocarbon such as paraffin or diesel oil, in the gas chamber of the reactor vessel, is enriched in the surface layer of the powder particles, whereupon the powder particles acquire a high content of carbide- bound carbon and a low content of oxygen in the surface layer. 5. A process as claimed in any of claims 1-4, characterized in that the sedimentation equipment (12) comprises at least two sedimentation tanks with associated wet containers. 6. An atomization plant for the manufacture of atomized metal powder comprising a reactor vessel (1), a casting box (2), a powder container (9) and sedimentation equipment (12) for producing atomized metal powder, characterized in that t h e atomization plant comprises a reactor vessel (1) having at least one primary nozzle (4) for the atomizing medium and at least one secondary supply arrangement (5) for coolant in the upper part of the reactor vessel, the bottom of the reactor vessel being conical to enable metal powder to be fed out to a powder container (9), and suction means (10) arranged in the lower part of the reactor for the removal of coolant and finer powder particles to said sedimentation equipment (12), the latter comprising at least two sedimentation tanks with associated wet containers, recirculation means leading from the sedimentation equipment (12) to return the coolant to said reactor vessel. 7. An atomization plant as claimed in claim 6 , characterized in that said secondary supply arrangement (5) comprises an annular extruder. 8. An atomization plant as claimed in claim 6 , characterized in that a sedimentation tank and a wet container are dimensioned to contain the entire quantity of coolant required to cool a full charge of powder. 9. An atomization plant as claimed in claim 6 or claim 7, characterized in that a liquid lock (7) is arranged in the reactor wall for evacuation of the overpressure formed when the atomizing medium is carbonized. 10. Atomized metal powder produced in an atomization plant comprising a reactor vessel (1), a casting box (2), a powder container (9) and sedimentation equipment (12), characterized in that the metal powder particles have increased content of carbide-bound carbon in the outer layer obtained by enrichment of carbon obtained from carbonization of some of the atomizing medium, which preferably consists of acyclic and /or isocydic hydrocarbon such as paraffin or diesel oil, which is introduced through one or more primary nozzles (4), and in that the powder particles are spherical in shaped as a result of coolant being supplied at low pressure via at least one secondary supply arrangement (5). 11. Atomized metal powder as claimed in claim 10, characterized in that said metal powder contains steel with an increased content of carbon-bound carbon and low oxygen content in its surface layer. 12. Atomized metal powder as claimed in claim 10 or claim 11, characterized in that the size distribution of said spherical particles is >150μ, 150-20μ and <20μ, preferably >100μ, 100-20μ and <20μ. 13. The use of atomized metal powder as claimed in any of claims 10-12, characterized in that said metal powder is used as coolant in the manufacture of steel, preferably continuously cast steel. 14. The use of atomized metal powder manufactured in accordance with the process as claimed in any of claims 10-12, characterized in that said metal powder is used for manufacturing tool steel. 15. The use of atomized metal powder manufactured in accordance with the process as claimed in any of claims 10-12, characterized in that said metal powder is used as additive in steel powder mixtures for powder-metallurgy production in a content of approximately 10%. 16. The use of atomized metal powder manufactured in accordance with the process as claimed in any of claims 10-12, characterized in that said metal powder having a particle size less than 150μ is used as additive in steel powder mixtures for powder-metallurgy production.

Inventors: AXMIN ERIK, LARSSON RUTGER, AXMIN, ERIK, LARSSON, RUTGER
Applicants: AXMIN ERIK, LARSSON RUTGER, RUTGER LARSSON KONSULT AB, AXMIN, ERIK, LARSSON, RUTGER
Application year: 1997
Main classification: B22F9
IPC codes: B22F9