WO2008094036A1

WO2008094036A1 - Device and method for purifying a metal flow

Info

Publication number: WO2008094036A1
Application number: PCT/NL2008/050052
Authority: WO
Inventors: Andrey Nikolaevich Turchin; Dmitry Georgiyevich Eskin; Laurens Katgerman
Original assignee: Stichting Materials Innovation Institute (M2I)
Priority date: 2007-01-31
Filing date: 2008-01-29
Publication date: 2008-08-07
Also published as: NL2000461C2

Abstract

Device for purifying a metal flow comprising a vessel (2) having an inlet conduit (3) and an outlet (8). Swirl in the vessel is generated by embodying the inlet conduit as a spiral. The outlet opening iof the vessel is surrounded by an upstanding edge (6) or sleeve which defines an outer cavity in which flow reversal takes place and an inner cavity (5) connected to said outlet opening (8). At the free extremity of the upstanding edge a core (4) protruding from the top of the vessel protrudes into the upstanding edge thus leaving a gap (15) therebetween for admitting discharge of the metal flow. Through these measures separation of solid particles from a metal flow such as an aluminium alloy is optimised.

Description

Device and method for purifying a metal flow

The present invention relates to a device for purifying a metal flow comprising an inlet conduit, a separation vessel connected to said conduit, said separation vessel having an outlet for a purified metal flow. Such a device is known from

WO2004/001078 of Netherlands Institute for Metal Research. A cyclone is disclosed in this PCT application having an inlet conduit with off-center opening into the cyclone. A swirl is generated for separating impurities from the metal flow.

The level of the impurities in casting practice is crucial for some cast products, e.g. intended for foil production, and determines their quality. The moment the molten metal containing a high fraction of inclusions reaches the casting stage, for example the hot top during DC (direct chill) casting, the removal of the inclusions becomes impossible, especially in case of close densities of the melt and particles.

The entrapped oxides in the form of films and spherical particles are the most common incident in the solidified metal. Oxides can be generated by furnace handling operations, metal distribution system and surface disturbances in the metal flow with the free surface (aluminium oxides).

The impurities in the solidified metal determine the minimum possible thickness of the final product and formation of the porosity, which can lead to further defects such as hot tearing, cold cracking etc. It is also found that for most metallic systems the mechanical properties dramatically decrease if the high fraction of the inclusions is present in the metallic matrix.

The level of oxides and inclusions in the molten metal in to-day's casting practice is controlled by filter boxes and degassing units. However, the application of such systems, i.e. filter boxes, can be limited since during the transfer of the molten metal through these systems pick-up of earlier removed impurities can occur due to varying efficiency of the filter ("filter aging") or fluctuations in the flowing melt. Moreover, the expensive filters have to be exchanged regularly.

The economical and environmental aspects of the metal industry require the increasing part of the scrap recycling. Obviously, the scrap contains high fractions of insoluble impurities which have to be removed at the initial stage of the production chain. Presently, the design of the separators and apparatus for the particle removal is adapted to the different media and based on the tangential introduction of the media enriched with the impurities and inclusions at high velocity into the chamber constructed as a cone. Thus, the medium is forced downwards under the gravity and centrifugal forces. The impurities are collected in the tapered section. The well-known example in the state of the art is a cyclone for removing dust particles or other impurities from gases (US 3,790,143 A). In this case the centrifugal forces are created due to off-axial introduction of gas into the cyclone. US-A-6024874 discloses a cyclone -pipe separator device having a central core portion for guiding the liquid flow discharged.

The aim of the present invention is to provide the method and device for insoluble impurities removal from the flow of the molten metal in a wide range of flow velocities and particle sizes. These impurities can be represented by solid particles of metal oxides, intermetallics formed in the bulk flow or solid particles entrapped into flow during melt transport, etc.

This aim is realised with a device as described above in that said inlet conduit comprises a helical-like section with a substantially vertical axis, spanning an arc of at least 180° and a pitch (P) of 0.8-1.2 of the mean diameter (2R) of said helicoid. This aim is realised with a device having the features of claim 1.

Through the helical-shaped inlet conduit a helical flow is created giving an improved spiral movement of the flow. The filter particles are more easily separated. The filtered media can be any liquid, in particular but not necessarily molten metals. Such metals can have a single melting point or a melting range within a wide temperature range. The invention can be applied to semi- or continuous molten metal casting operations. Impurities, such as oxides, slag inclusions, etc. can be removed with the subject device. Because of the helical shape of the inlet conduit the metal flow will obtain both a circular movement and a vertical movement together with sufficient speed at entering of the separation vessel. In this way a centrifugal effect to separate impurities is optimized.

According to a preferred embodiment of the invention the height of the helical conduit is ¹A-¹A of the total height of the device. The vessel is preferably a cyclone purifier and according to the invention the outlet opening thereof in the lower part is delimited by an upstanding edge or sleeve. Between this upstanding edge and the inner surface of the vessel a cavity is defined. Metal circulating along the inner wall of the vessel will be reversed in flow direction near the lower part of the cavity and an internal upward flow along the outer wall of the upstanding edge or sleeve is obtained wherein the liquid metal flows over the edge into the outlet opening.

To further optimise the separation of impurities according to a preferred embodiment of the invention a downwardly extending core is provided, preferably tapering in downward direction. Preferably this core extends within the area of the opening delimited by the upstanding edge leaving a gap between the overflow of the edge and the related core area through which metal flows through the outlet opening.

The cavity described above can be provided with a drain for draining impurities. Further efficiency can be obtained if this drain is connected through the inlet of the device giving recirculation of metal or other liquid to be purified. Part of the drain will, of course, be discharged.

The device is preferably constructed as a box with an inlet window connected with the helical conduit. Due to the helical introduction of the molten metal into the cavity, an adequate degree of the spiral motion is initiated in the cavity around the centred core to the molten metal which is already fed in. The cavity has a cylindrical shape of the specific length. Additionally, the central core has certain dimensions variable in respect to the inner diameter of the cavity and that of the outlet window and dependent on the imposed requirements of the device. The length of the core is preferably % the cavity length. The ratio of the minimum to maximum diameter of the core is preferably % . The core is fixed to the cavity wall on the one end and on the other has a convex tip with the rounding-off radius preferably two times smaller than the minimum diameter of the core. The diameters ratio of the core is responsible for the pressure change at the outlet window and the resulting flow pattern in the present device. The length of the sleeve is preferably % the cavity length. Due to specific flow pattern in the device the particles are either driven into the corner at the bottom of the cavity or suspended in it. Preferably axes of the block, the cavity, helix axis of the helical channel, core, and chamber and outlet tube are aligned in operation and parallel to the gravity direction.

The pressure drop is of great importance for all filtration and separation systems. The main flow direction in the instant invention is basically gravity direction, vertically downwards and the velocity at the outlet windows can be controlled due to changing of the outlet pipe diameter within a reasonable range without significant reduction in the efficiency.

The flow of the molten metal, depending on the inlet boundary conditions, is either laminar or turbulent. The flow pattern in the device, e.g. its velocity, can be controlled by different ways, such as the pitch length of the helical channel, dimensions and shape of the core, and the chamber between outlet pipe and the inner wall of the cavity.

The structure according to the initial design does not have moving parts. One advantage of the invention is the possibility to use the device for particle removal from the molten metal in a wide flow velocity (>0.1 mm/s) and particle size (<0.01 m/m) ranges.

An advantage of the instant invention is the particle removal of different densities and sizes using a controlled and predicted flow path as dictated by the inlet velocity and time calculated to predict the efficiency.

Another advantage of the present invention is the fixed helical channel of a specific dimension and the lack of the moving parts that enables one to use the predetermined process conditions to predict the degree of efficiency.

Additionally, the advantage of the invention is the lack of the free surface and contact with the atmosphere during the molten metal operations and, for alloys, better mixing of the molten metal due to gravity and centrifugal forces.

A further advantage of the instant invention is possibility for its application in many casting processes, such as direct-chill casting, sand casting, continuous casting etc. The invention will further be elucidated by referring to the enclosed drawings, wherein:

Fig. 1 schematically shows the device according to the invention; Fig. 2 shows a cross-section through line II-II of Fig. 1; and Fig. 3 shows an assembly of the devices according to the invention. In Fig. 1 the device according to the invention is generally denoted by 1. In this embodiment it has been realised by shaping in a block of ceramic material and it will be understood that the structure can be made from any non-ceramic material and that the device can be obtained by assembling of separate components. The filter device comprises an inlet conduit 3 which opens in a separation vessel 2 having an outlet 8. A vertical axis has reference numeral 9. A core 4 is provided tapering in downward direction in separation vessel 2. At the bottom of the separation vessel 2 an upstanding edge or sleeve (bush) 6 is provided leaving a cavity 5 between the outer surface of the sleeve and the inner surface of the separation vessel (see also Fig. 2). At the lower end of cavity 5 a drain 16 is provided. The upper part of the sleeve 6 functions as overflow. From the figures it is clear that the core 4 extends within this overflow leaving a gap 15 for the liquid to be discharged. The inlet conduit 3 has a special shape according to the invention. It is embodied in a helical or spiral-like fashion extending around vertical axis 9. The height h of the spiral is ¹A-¹A of the total height H of the device. The spiral spans an arc of at least 180° around vertical axis 9 and the pitch (full revolution) of the spiral is about 0.8-1.2 of the mean diameter 2R of the spiral. In this way optimum flow conditions of the metal emerging from the inlet conduit in the vessel are obtained. This flow is a helical flow and is indicated in the drawing with a dotted line 12. Through the difference in height over the inlet conduit and tangential movement enforced by the helical shape of the conduit 3, the metal will flow with high velocity along the inner wall of the vessel 2 promoting separation of impurities. As is shown in Fig. 2 the rotational spiral-like movement will continue until the flow will reach the bottom of the cavity 5. There reversal of the flow is effected in such a way that inside the spiral flow of the downwardly moving metal an inward flow of purified metal along the outer surface of the sleeve 6 is realised. This is indicated with line 14. Regarding the overflow the metal will flow in the gap 15 between the inlet conduit 3 and the inner surface of the sleeve 6 and move in downward direction. This purified metal flow or other flow can be further processed. Impurities will settled in the lower part of the cavity 5 and can periodically be discharged through drain 16.

In Fig. 3 an assembly 17 of three devices 1 according to the invention is shown. The outlets 8 are connected to a common outlet 19. The drains 16 can also be connected to a common discharge and a conduit 20 is provided for an optional recirculation of the drained liquid to the inlet conduits 3. It is also possible to arrange a number of devices 1 according to the invention in series, which means that the outlet 8 of one device is connected to the inlet 3 of the next device.

Molten metal flow 10 enters the inlet window 11 of the helical channel 3. Flowing through the conduit the stream of the molten metal acquires the spiral motion around the core 4. The resulting flows under steady state conditions are composed of the inertial, helical extending, peripheral flow (dashed line) 12, strong and free of impurities flow 13 into the outlet pipe 8, and the circulated potential flow 14 in the opposite direction from the chamber 5 into the outlet pipe 8. The foregoing description discloses and describes a preferred embodiment of the present invention. Obviously, modifications and design variations of the present invention are possible, depending on the metal, casting process, and process conditions. It is, therefore, to be understood that one skilled in the art will recognize from such discussion and from the accompanying drawing and claims, the variants of the instant invention can be made therein without departing from the true spirit of the invention as defined in the following claims.

More in particular rights are claimed for subclaims as such, i.e. without the combination with one of the previous claims and/or main claim.

Claims

1. Device (1) for purifying a metal flow comprising an inlet conduit (3), a separation vessel (2) connected to said conduit (3), said separation vessel having an outlet opening (8) for a purified metal flow, said inlet conduit comprises a helical section with a substantially vertical axis (9), spanning an arc of at least 180° and a pitch (P) of 0.8-1.2 of the mean diameter (2R) of said spiral wherein adjacent to said outlet opening (8) an upstanding edge (6) extending from said outlet opening is provided and wherein a cavity (5) is delimited between said upstanding edge (6) and the inner surface of said vessel (2), and a central core (4) is provided near the bottom end of the vessel leaving a gap (15) between said upstanding edge (6) and the inner surface of said vessel (2).

2. Device according to claim 1, wherein the height h of said helical conduit is

V4-V2 of the total height H of the device.

3. Device according to one of the preceding claims, wherein the vessel is a cyclone purifier.

4. Device according to one of the preceding claims, wherein said cavity is provided with a drain (16) for impurities.

5. Device according to claim 4, wherein said drain (16) is connected to said inlet conduit (3).

6. Device according to one of the preceding claims in combination with claim 4, wherein said upstanding edge (6) extends over less than 50 % of the total height H of the vessel.

7. Device according to one of the preceding claims, wherein the core (4) tapers in downward direction.

8. Device according to one of the preceding claims, wherein said core extends within said upstanding edge (6) leaving a gap (15) between said core and the inner surface of said upstanding edge (6).

9. Method for purifying a liquid flow of metal comprising entering said flow in a separation vessel at imparting a downwardly directed spiral movement thereon, impinging said flow to the bottom end of said vessel and reversing the flow thereof in upward direction, wherein said upwardly reversed flow (14) is discharged via an overflow.

10. Method according to claim 9, wherein the upwardly directed flow is within said downwardly directed spiral movement.

11. Method according to claims 9 or 10, wherein said metal is aluminium based.