Note: Descriptions are shown in the official language in which they were submitted.
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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 ef the manufacturing process fo: 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
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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
S 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 a
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
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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.
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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
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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
5 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 f 1 o w
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.
