DRYING OF ATOMIZED METAL POWDER

Abstract: Metal powder, particularly iron/steel powder with low oxygen content, is atomized in an oxygen-free atmosphere by means of a hydrocarbon in liquid form such as paraffin, and the moist powder, still hot (60-80 DEG C), is transferred to a container (1) and supported therein on a screening member (2). A negative pressure is first applied below the screening member (2) so that liquid is drawn from the powder mass. An inert gas, substantially free from oxygen, is then circulated through the powder mass, whereupon vapour absorbed by the gas is condensed before the gas is recirculated to the container (1). A circulation circuit (70) for liquid atomizing medium joins the gas-circulation circuit (20) at a liquid jet pump (30).

Description: Drying of atomized metal powder The invention relates to a process for drying and possibly cooling atomized metal powder which is moist from a liquid used during the atomizing process, and which is surrounded by an atmosphere typical of manufacturing processes for metal powder, e.g. a substantially inert atmosphere, and equipment for performing the process. When manufacturing metal powder, e.g. steel powder with the lowest possible oxygen content and possibly with a selected relatively high carbon content, a stream of molten metal is atomized with the aid of an atomizing agent. The atomizing agent may be in gaseous or liquid form and may comprise a liquid hydrocarbon such as paraffin, fuel oil class 1, water or the like, for instance, in an environment substantially free from oxygen. The powder formed falls down into a pool of coolant which may consist of the atomizing agent. The powder is tapped to a transport container and the powder, usuall with increased temperature, e.g. 60-90°C, is then dried in a separate drying operation which constitutes an important part of the manufacturing process for many types of metal powder, particularly iron/ steel powder having low oxygen content. The powder is kept in a protective atmosphere (i.e. the typical atmosphere mentioned above or substantially inert atmosphere) until the risk has been substantially eliminated of the powder being adversely affected by the surrounding atmosphere. According to conventional drying technology the moist powder is kept in an oxygen-free atmosphere of nitrogen, where the nitrogen may be of particularly oxygen-free quality in order to prevent oxidation. According to the known technology, the atomizing liquid is first allowed to run off the powder mass which is then tumbled by heated inert gas which is allowed to circulate through the tumbling powder mass so that the atomizing agent is vaporized and the vapour accompanying the inert gas is condensed and separated from the gas before the latter is recirculated. However, the conventional technology has been found to require extensive and consequently expensive drying equipment, as well as entailing relatively high energy consumption. The dried powder also has a high temperature which means that it must be cooled before it can be exposed to the surrounding atmosphere, and such powder cooling is expensive since it requires a cooling arrangement and since the thermal energy is cooled off. The reason the powder now has to be cooled is a desire to avoid subsequent oxidation of the powder when it is emptied from the drying equipment, and also that the screening cloths of conventional screening means are attached by material which melts at low temperature, e.g. 60°C. An object of the invention is to provide a drying method that can be performed in a relatively simple apparatus and with relatively low energy consumption, preferably so that the dried powder immediately acquires a relatively low temperature and can preferably b e immediately subjected to subsequent treatment, such as screening using conventional screening means. The object of the invention is achieved with the technology defined in the appended independent claims. Embodiments of the invention are defined in the appended dependent claims. Additional objects and advantages of the invention are revealed in the following or will be obvious to one skilled in the art upon a study of the following description of one embodiment of the drying equipment. Central features of the process according to the invention are thus that the moist metal powder is contained in a container on a screening member disposed transversely in the container, that a vacuum is applied to the lower side of the screening member in order to draw off some of the liquid atomizing medium, that after the liquid has been drawn off, inert gas such as nitrogen gas, is circulated through the container in order to absorb residual moisture in the form of vapour from the powder mass, and that the vapour is removed from the gas forced through the container prior to recirculation in the container. The liquid moistening the powder can be drawn off by means of a liquid jet pump (ejector pump). Such an ejector pump can also be used to subsequently withdraw vapour and gas from the container and to circulate the inert gas. The ejector pump comprises a circulation circuit for the liquid driving the pump. The driving liquid may consist entirely or partly of the atomizing medium used for manufacture of the powder. The vapour sucked out by the pump is condensed in the liquid driving the ejector pump. The liquid-circulation circuit of the ejector pump may also contain a heat exchanger allowing the circulating liquid to be heated or cooled. The circulation circuit may also include a holding tank. In such a holding tank an upper gas space can be established in conventional manner, so that inert gas is permitted to separate from the liquid of the circulation circuit in the holding tank, and rises to the upper part of the holding tank so that inert gas can be conveyed for recirculation to the powder container. The inert gas pipe may contain a heat exchanger to enable heating or cooling of the inert gas before it is reintroduced into the powder container. A negative pressure is preferably established in the container in order to accelerate vaporization of the atomizing medium, whereby the temperature of the powder mass can also be reduced to a desired level. If desired the liquid can first be withdrawn from the container with the aid of a suitable liquid suction pump, such as a membrane pump. After which an ejector pump can be used as described above. With certain particle size gradients of the atomized powder a relatively "dense" cake may be formed on the screening member and according to one embodiment of the invention the cake is broken up in order to facilitate a flow of inert gas through the powder mass, thus facilitating drying of the powder. An important feature of the invention is that the outflow, particularly the outflow of vapour from the container, is combined with the flow of driving liquid in an ejector pump so that the vapour leaving is condensed in the flow of driving liquid, while the inert gas can easily be separated for recirculation. Since the drying process is initiated with suction of liquid atomizing medium from the powder mass, the residual moisture in the powder mass will be low and the energy required for the final drying process is reduced and thus also the heat of the powder itself (possibly in combination with a negative pressure) can provide the vaporization energy necessary for final drying. After final drying, therefore, the temperature of the powder is sufficiently low to enable additional cooling to be partly or entirely eliminated, which would otherwise be needed to prevent subsequent oxidation of the powder if exposed to the surrounding atmosphere. Additional objectives and advantages of the invention are revealed in the following or will be understood from a study of the following description of one embodiment of a drying apparatus. Figure 1 shows a wet container 1 with a screening plate 2 disposed transversely in the container and carrying a metal powder mass 3. A stirrer 4 is also shown schematically to break up the powder bed 3 if necessary. The container has upper inlets 10, 14 and a lower outlet 12 with closing valves 11, 15 and 13, respectively. The container 1 can be connected to a powder-atomizing plant and receives wet powder produced there via the inlet 14. The closed container 1 can then be connected into a gas-circulation circuit 20, with valves 21, 22, as shown in the drawing. The gas-circulation circuit 20 contains an inert gas such as nitrogen with a low content of oxygen and other damaging compounds. The circuit 20 is supplied with inert gas from the gas source 23 as needed. The circuit 20 includes a holding tank 25 for liquid atomizing medium, and a suction pump 30. The suction pump, e.g. an ejector pump, first withdraws liquid atomizing medium from the powder mass 3 and conveys this liquid to the holding tank 25. The particle filter, not shown, separates out any particles that may have accompanied the liquid. The holding tank 25 has an upper space 26 which thus contains inert gas. The tank 25 naturally also contains a mass 27 of liquid atomizing medium and this liquid mass 27 can be kept at a constant level by means known per se. The liquid atomizing medium is circulated from the tank 25 via a pump 60 and a cooler 61 to the ejector pump 30, causing this to produce a negative pressure in the outlet side of the container 1. The quantity of liquid in the powder can be reduced to an optimally low proportion by means of suction and the residual moisture departs in the form of vapour, to the gas flow circulating in the circuit 20 with the aid of the pump 30. The vapour accompanying the gas flow is condensed in the cooled atomizing liquid circulating through the circulation circuit 70. The partial pressure in the lower side of the container 1 facilitates vaporization of the remaining liquid in the powder mass 3, as well as lowering the temperature of the powder mass 3. If desired, however, a heater 80 may be connected in the gas-circulation circuit 20 upstream of the container 1, if the powder itself is not sufficiently hot to vaporize the remaining moisture at the prevailing pressure. The drawing shows an ejector pump 30 both for removal of liquid and circulation of gas. However, it should be evident that a liquid suction pump, e.g. a membrane pump, can be connected at least temporarily in parallel with the ejector pump 30, for instance, to provide optimal liquid suction conditions. Furthermore, the ejector pump 30 may be replaced by another type of pump for circulating the gas. An important feature of the embodiment shown is that the gas- circulation circuit and the liquid-circulation circuit have a common section so that vapour accompanying the gas flow is condensed in the liquid, preferably cooled, and that the gas circuit and liquid circuit are preferably joined via an ejector pump.

Claims: CLAIMS 1. A process for drying atomized metal powder contained in a protective atmosphere and moist from a liquid used during the atomizing process, characterized in that the moist metal powder (3) is contained in a container (1) on a screening member (2) disposed transversely in the container, in that a vacuum is applied to the lower side of the screening member (2) in order to draw off some of the liquid, in that after the liquid has been drawn off, protective gas is circulated through the container in order to absorb vaporized liquid from the powder mass, and in that the vapour is separated from the protective gas circulating through the container prior to recirculation in the container. 2. A process as claimed in claim 1, characterized in that the liquid is drawn off by means of a suction pump such as a membrane pump. 3. A process as claimed in claim 1 or claim 2 , characterized in that a liquid jet pump (30) is connected to the lower part of the container (1) in order to establish negative pressure below the screening member (2), and in that the liquid pump is driven by a circulating liquid flow, the vapour condensing in the circulating liquid flow of the ejector pump, said flow consisting partly or entirely of the liquid with which the powder was moistened. 4. A process as claimed in claim 3, characterized in that the liquid-circulation circuit of the pump (30) includes a holding tank (25) which is kept only partially filled with liquid in order to permit separation of the protective gas accompanying the circulation liquid, said gas being withdrawn from the holding tank and circulated to the container. 5. A process as claimed in any of claims 1-4, characterized in that the flow of protective gas is heated (80) before being returned to the powder container (1). 6. A process as claimed in any of claims 1-5, characterized in that the cake of moist powder mass (3) formed on the screening member (2) at withdrawal of the liquid is broken up (4) in order to facilitate circulation of the protective gas through the powder mass. 7. Equipment for drying atomized metal powder contained in a protective atmosphere in a container (1) and moist from a liquid used during the atomizing process, characterized in that the moist metal powder (3) in the container (1) rests on a screening member (2) disposed transversely in the container, in that said equipment includes a pump member (30) to draw liquid from the powder mass (3), and in that the pump member is arranged to circulate protective gas through the container (1) to take with it vapour stripped from the powder mass, and in that condensors (25, 26, 27) are arranged to condense the vapour accompanying the protective gas so that the protective gas is substantially freed from the vapour before being recirculated to the container (1). 8. Equipment as claimed in claim 7, characterized in that the pump member comprises a circulation circuit (70) for a liquid, in that the upper and lower parts of the container (1) are connected to a circulation circuit (20) for the protective gas, in that both circuits (20, 70) join each other at the pump member, which is in the form of a liquid jet pump (30), so that the vapour accompanying the protective gas is condensed in the liquid-circulation circuit (70) of the pump member. 9. Equipment as claimed in claim 8, characterized in that the liquid in the liquid-circulation circuit of the liquid jet pump consists entirely or partly of the liquid with which the powder was moistened.

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, F26B5, F26B21