The present invention concerns an apparatus and method for the removal of unwanted inclusions from metal melts by filtration.

It is generally known to remove small inclusions from molten metal such as molten aluminium by filtration. A typical material used for such filters is porous ceramic or refractory material, commonly referred to as CFF (Ceramic Foam Filters). These CF filters are not easily wetted by the molten metal and since such materials have relatively fine pores, considerable difficulties are encountered in initiating the flow of metal through the filter (priming the filter). It is therefore generally known to use deep filter boxes to generate sufficient metal head by gravitation to force the metal through the filter.

Japanese patent application, JP 60-5829 relates to a filtering method which proposes the use of a vacuum to prime a CF filter and where the filter is provided in the bottom of an evacuation vessel in vacuum tank. The vacuum tank is set under vacuum by a vacuum pump and metal is thereby forced to flow from the vessel through the filter and into the vacuum tank and further through an outlet to a casting site. Once the flow is initiated the vacuum pump is halted and the metal flows by itself based on a metal head (gravity). EP 1 735 120 describes a method and apparatus for the initiation of flow of metal through a filter in an in-line molten metal filtration unit having a porous ceramic or refractory filter mounted horizontally in a filter box. The filter box is provided with an inlet and outlet for the molten metal. When operating the apparatus, metal is supplied through the inlet to fully cover the filter and the outlet is closed whereby the box set under vacuum such the metal is forced downstream through the filter. Then as soon as the metal flow is initiated, the vacuum is taken off and the outlet is opened such that the metal may flow through the filter on the basis of gravity. Thus, the vacuum is just used to initiate the metal flow through the filter.

The use of a CF filter involves extra costs. These costs are partly linked to the filter itself that must be replaced, normally after each casting operation. There is also some cost related to heating the CF filter and the filter box. In addition, the filter boxes according to the prior art as referred to above have a reservoir of metal below the filter which must be drained after each casting cycle. For a large single filter box the amount of drained metal may amount to 5-600 kg per cast. The cost associated with remelting of metal is typically 1000 NOK / ton, which gives 10-15 NOK per ton finished goods (depending on charge size).

For a filter with small pore sizes (> ~ 50 PPI (pores per inch)) it may also be problems getting the entire filter activated (primed) during startup and thus enable it to function optimally. Priming of the entire filter is often linked to correct preheating, but can also be linked to obtaining high enough pressure on the initial metal that goes through the filter. Possible solutions suggested are overpressure in front of the filter or under-pressure behind the filter as is referred to in the prior art solutions mentioned in the above cited JP and EP references. Another known solution is a CF apparatus with a filter solution using an electromagnetic field that "pushes" the metal through the filter. This solution is quite expensive and is also encumbered with poor metal draining.

With the present invention is provided a CF filter apparatus that reduces operational cost since it is self-draining and needs no metal removal after each casting operation. Further, the new CF filter concept provides very good priming of the filter.

The invention is characterized by the features as defined in the independent apparatus claim 1 and method claims 10 and 1 1 .

Dependent claims 2 - 9 and 12 - 14 define preferred embodiments of the invention.

The invention will be further described in the following by way of examples and with reference to the drawings where: Fig. 1 shows in perspective view a cross section of a CF filter device or apparatus according to the invention,

Fig. 2 shows a partly longitudinal view of the CF filter apparatus as shown in Fig.

1 , but with the inlet and outlet connected to a parallel provided launder for the supply of metal to and from the filter apparatus,

Fig. 3. shows in smaller scale a schematic cross section of the filter apparatus and launder as shown in the previous figures. Figs. 4 - 8 show schematically and also in smaller scale top cross sections of sequences of operational steps of the apparatus according to the invention.

Fig. 9 shows in cross section and partly perspective view a further embodiment of a CF filter apparatus according to the invention as shown in Figs. 1 and 2.

Referring to Figs. 1 and 2, the CF filter apparatus according to the present invention includes a container or box construction 1 with an outer shell or casing of metal and an inner thermally insulated interior cladding or wall construction made of heat resistant insulation and refractory material (not further shown). A removable lid 2 is provided (preferably hinged to the container) on top of the container to keep the container sealed off (air tight) during operation of the CF filter 7. The container 1 has an inlet chamber 3 with an inlet opening 4 (the inlet chamber should be as small as possible, and an outlet chamber 5 with an outlet opening 6 where the CF filter 7 is mounted.

The inlet chamber 3 and outlet chamber 5 are provided side by side within the container 1 , being split by a partition wall 16 extending from the bottom of the container and upwardly, to a preset height (approximately 2/3) of the container height.

Referring to Fig. 2, the container is connected in parallel with a metal supply launder 10 via transversal metal launder stubs 8, 9 respectively provided between the inlet 4 and outlet 6 openings and the metal supply launder 10. To control the metal flow in the metal supply launder, two dams or valve discs (butterfly type valves) are provided in the launder 10, one dam 1 1 by the outlet of the container 1 and the other dam 12 in the launder 10 between the two launder stubs 8 and 9.

The inlet and outlet openings of the container are further provided with closures or stoppers 13, 14 respectively (not further described).

Preferably a programmable logic control (PLC) is provided to control the operation of the filter apparatus according to the invention. This PLC unit will not be further described and is not shown in the figures since it is regarded as forming part of the general knowledge base for the skilled person within this field.

Fig. 3 shows as stated above in smaller scale a schematic cross section of the filter apparatus and launder as shown in the previous figures 1 and 2. As can be seen, the container 1 at its upper part is connected to an ejector device or other air evacuating means 15 to enable evacuation of air from the container. Thus, by closing the lid 2 it is possible to set the container under vacuum and thereby draw liquid metal from the launder and into the container and further raise the metal level in the container as will be explained below. The working principle of the filter apparatus according to the invention is, with reference to Figs 4 - 8, as follows:

When using the filter apparatus in connection with a metal casting operation, metal is supplied from a metal reservoir such as a molten metal holding furnace by the launder 10 to the filter container 1 . Referring to Fig. 4, both the inlet 4 and the outlet 6 openings are closed by means of the stoppers 13, 14 respectively during filling of the launder. The ejector is then started and air is thereby evacuated form the container as soon as the metal level is above the openings.

Further referring to Fig. 5, the priming operation is now performed by opening the stoppers 14 at the inlet 4 and 13 at the outlet 6 of the filter container, and the under-pressure (vacuum) created by the ejector lifts the metal until it meets the underside of the filter 7 in the outlet chamber and to a position slightly above the top of the filter in the inlet chamber. Then the inlet is closed with the stopper 14 and the underpressure is further increased until the metal is forced backwards up through the filter.

By evacuating the filter container before opening the outlet, the priming operation can be controlled by adjusting the outlet opening 6 with the stopper 13. In this way the priming operation can be performed faster and be better controlled.

The under-pressure above the filter determines the force applied to the metal. The underpressure can then be adjusted to give good priming also for CF filters with high PPI values. After the filter has been primed the inlet 4 is opened by withdrawal of the stopper 14 as is shown in Fig. 6. Then, referring to Fig. 7, when the metal level inside the filter container has reached a preset level above the partition wall 16, the dam 2 (see in addition Fig. 2) between the inlet and outlet openings is closed whereas the dam 1 1 close to the filter container is opened. The metal is now forced to go through the CF filter and the metal level is held at its pre-set position by the vacuum caused by the ejector or vacuum source 15. The height difference (metal head) between the metal outside the inlet opening and the metal outside the outlet opening is the driving force to get the metal through the CF filter. At the end of the casting operation the metal level inside the container 1 is gradually lowered by reducing the under-pressure. When the metal has reached a certain level the dam 1 1 between the inlet 4 and the outlet 6 is opened, and all the metal in the box is released to the casting pit (not shown).

The metal in the inlet chamber 3 (which preferably should be as small as possible) will either go backwards or towards the casting pit, however it will not enter the part of product that goes to the customer.

The stopper 13 at the outlet opening 6 is mainly there to avoid oxides and other inclusions at the top of the melt to enter the filter container and cause problems in the priming operation.

As previously mentioned, it could be an advantage to evacuate the filter container prior to opening the stopper 13 at the outlet 6. The priming can then be performed faster and with more under-pressure (vacuum) as the metal enters the filter openings.

One option which may ease the priming operation would be to flush the filter box with argon gas prior to the evacuation process. This may reduce the oxidizing of the metal that enters the filter box, which probably is beneficial in the priming operation.

As the priming is done in the reverse direction it is important that the CF filter is properly fastened to the interior of the container (not further shown).

A simpler embodiment of the filter box can be obtained without the stoppers 13 and 14. In that case it could be beneficial to have dams (not shown) at the start of the transversal metal launder stubs 8 and 9 to release metal to the filter box only from the bottom of the launder by gradually rising the dams. This will prevent oxides and other non-metallic particles at the top of the melt to enter the filter box. As soon as the metal level is above the inlet and the outlet openings 4, 6, the ejector 15 starts to evacuate the air inside the box 1 drawing the metal upwards in the inlet and outlet chambers 3, 5. The metal will stop against the bottom of the filter 7 in the outlet chamber because there is a resistance for the metal to enter into a filter with small pores. In the inlet chamber the metal will continue to rise as the under-pressure increases. When the height difference between the bottom of the filter 7 and the metal inside the inlet chamber 3 has reached a certain level, the pressure on the bottom of the filter will be high enough to prime the filter. The maximum pressure for priming that can be obtained with this solution will be the height difference between the bottom of the filter 7 and the top of the partition wall 16. This difference will be limited by the operational difficulties in mounting and dismounting the CF filter 7 very far down in a deep outlet chamber. Thus, this solution may work well for relatively coarse filters where the necessary pressure to prime the filters are not too high. As an alternative embodiment, there could also be an option to make the inlet chamber larger and the outlet chamber smaller mounting the CF filter just above the opening of the inlet chamber. In that case the direction of the flow of the metal will be the same in the priming phase as in the operational phase. The drawback is that there will be more metal released towards the casting pit at end of the end of each casting operation that has not gone through the CF filter.

Preheating of the CF filter and the filter container prior to the metal filtering operation is important. This can either be done through the inlet or the outlet opening (not further shown) or there could be some arrangement for preheating in the lid of the filter container (neither not shown).

As an alternative embodiment, a double filter box can be made by having one inlet chamber and two outlet chambers with separate filters. With two outlet openings and corresponding stoppers the CF filters can be primed one by one. Without stoppers in front of the openings the filters will be primed in a similar way as described for one outlet chamber.

In a further embodiment, there is also an option to not empty the filter container between each casting operation, but instead reuse the CF filter for several casting cycles. This can be done by closing both the inlet and the outlets and keeping the melt hot in between each casting operation. By keeping an under-pressure in the container it will be easier to avoid melt leakages in the inlet and outlet openings during the casting break.

The proposed way of priming according to the invention is very beneficial for dual filter options, one filter above the other, where it would be possible to have a coarse filter as an initial filtering step and a finer filter afterwards, and preferably with a gap in between. In a standard filter box there will very likely be air trapped in between the filters when the first metal goes through the upper filter and covers the lower filter. To avoid this, a tube may be provided in conjunction with the surface of the upper filter and up above the metal level to let the air in between the filters escape. When the priming of the lower filter is complete the tube can be removed.

With the current invention the lower filter with the smallest pores that is most difficult to prime will then be the first one to be primed. The whole priming operation with two filters will not require any manual handling as for the current filter box layout.

As to the application of two filters, it should be added that the invention is not restricted to the above described solution. Thus, it may be possible to provide the filters respectively in the inlet chamber 3 and outlet chamber 5. It may, within the scope of the invention, even be possible to use a combination of three or more filters if desired.

The principle of lifting the metal to avoid draining can also be utilised for Bonded Particle Filters (BPF) and Deep Bed Filters (DBF). This BPF filter medium is an aggregate of either SiC or AI2O3 granules, graded to a specific particle size distribution and then bonded together using a ceramic binder. In a further embodiment as shown in Fig. 9, the outlet chamber can be made much larger and deeper than what is indicated in the figures and be filled with AI2O3 balls 17 (or similar), as in a Deep Bed Filter (DBF).

Normally, a DBF is not emptied between casts since it contains several tons of metal that has to be drained and scrapped. Also, after emptying a standard filter box there will be severe oxidation of the fractions of metal left inside because of the air that will be available. With the present invention the metal inside the outlet chamber can be slowly released by adding argon gas to reduce the under-pressure and the chamber almost emptied to the casting pit towards the end of the cast. With the inlet and outlet openings sealed with metal and the temperature inside the filter box maintained during the break between two cast, there will be almost no oxygen and therefore very little oxidation of aluminium inside the filter box during the break. When the next cast starts only a fraction of the previous alloy is left inside the filter box. Therefore a change in alloy, at least within the same alloy system, should normally not be a problem.

This way it could be possible to operate a DBF also for a casting line with more frequent alloy changes. For operational performance it would be preferable having one filtering apparatus in operation at each casting line while one is being rebuilt with new AI2O3 balls. The lid would only need to be opened when the AI2O3 balls have to be replaced, e.g. approximately every 5000 tons (manual opening).