NO158105B - METHOD AND APPARATUS FOR EXHAUSTING AND FILTERING MELTED METAL. - Google Patents

Description

The present invention relates to outgassing of molten metal. Molten metal, especially molten aluminum, usually contains

in practice, entrained and dissolved pollutants both in gaseous and solid form, and these pollutants are harmful to the stopped end product. These contaminants can affect the stopped end product after the molten metal has solidified, so that material processing is made difficult or the end product becomes less formable or has poorer polishing and anodizing properties. The pollution can come from several sources. They can e.g. include metallic impurities, such as alkali metals and alkaline earth metals as well as dissolved hydrogen gas and surface films of oxide that are broken up and fast with the molten metal. Furthermore, there may be inclusions in the form of insoluble impurities, such as carbides, borides etc. or eroded refractory materials from the melting furnace and crucible.

A method for removing contaminants in gaseous form from molten metals is outgassing. This physical process involves injecting a flux gas into the smelt. Dissolved hydrogen penetrates the cleaning gas bubbles by diffusion through the melt of the bubbles, where it adheres to the bubble surface and is eventually absorbed into the bubble itself.

In this way, hydrogen is fed out of the melt with the help of the bubbles.

It is naturally highly desirable to achieve improved outgassing of molten metal in order to remove or reduce contaminants in gaseous form in the stopped end product, particularly in connection with molten aluminum and particularly when the resulting metal e.g. is intended to be used for decoration purposes, such as an ornamental object, or products for which high quality requirements are made, such as e.g. aircraft forgings and extrusions and thin foils. As mentioned above, contamination will cause a loss of valuable material properties, such as e.g. tensile strength and corrosion resistance, in the finished product.

Carefully carried out treatment processes, such as e.g. gas purification or melt filtration have, however, reduced the incidence of such material defects. Although such treatments usually reduce the incidence of such material defects to satisfactory levels, they have nevertheless been found to be inefficient and/or uneconomical. Flux gas processes are carried out in the usual way, such as e.g. ordinary hearth-flowing of flue gas has included the supply of such gas to a holding

furnace containing a certain amount of molten metal. This process involves the molten metal being held in the furnace for a specific time, while the flux gas is circulated, so that the amount of metal being treated is kept constant and efficient treatment can take place. Such a process has many disadvantages, including reduced efficiency and increased costs due to the long-term standstill in the holding furnace during the fluxing process, as well as above all the poor efficiency of the flux treatment due to poor flux gas coverage of the molten metal due to large bubble size and poor bubble dispersion in the smelting . Additional factors include the limitation of the treatment site to the holding furnace, which allows contaminants to re-enter the smelt for the plugging, as well as the high gas throughput that is a consequence of both the high flux required and the flux gas circulation area.

As an alternative to this portion-wise flux gas flow as stated above, certain flux processes have been used in the metal production paths themselves, which means that the flux process and the associated apparatus are placed outside the melting or holding furnace and often between the melting furnace and the holding furnace, or possibly between the holding furnace and the stopping point. Such an embodiment has helped to overcome the poor efficiency and the high costs which were the result of an unused melting furnace during the batchwise flux treatment, but has not been successful in terms of improving the efficiency of the outgassing process itself, since the large scale of the treatment units and the undesirably large required quantities of flue gas per unit molten metal, were both costly and harmful to the purity of the air.

Such a typical flow technique implemented in the production line is discussed in US patent document no. 3,737,304. According to this patent, a bed of "rock" is placed in a housing through which the molten metal will pass. A flux gas is introduced on the underside of said bed and flows up through the spaces between the stones by counterflow in relation to the flow direction of the molten metal. However, using a rental of porous "Stener" has an inherent disadvantage. The fact that the pores of the stones are so close leads to the fact that the gas bubbles that pass through the stones will coalesce on the surfaces of the stones, so that a relatively small number of large bubbles are produced instead of a large number of small bubbles. The resulting effect of the bubbles joining together is that the total surface area of the bubbles, which can be used for adsorption of hydrogen, is significantly reduced so that the efficiency of the outgassing becomes low.

An improved method and corresponding apparatus for outgassing

and filtering of molten metal in a production line is discussed in US Patent No. 4,052,198. This document shows an improved degassing and filtering of molten metal using an apparatus which makes use of a pair of replaceable elements of filter types placed one behind the other, as well as at least one flue gas inlet arranged between the elements. The flux gas is introduced into the smelter through its inlet and flows through the first of said filter plates in countercurrent contact with the smelter. This filter plate serves to split the flue gas into finely distributed streams to ensure extensive contact with metal

the forge. The filter plates used are made of porous ceramic foam materials which have proven effective in filtering molten metal for many reasons, including their excellent filtering properties which are a consequence of their uniform and adjustable pore size, low costs, as well as ease of use and replaceability. Such ceramic foam filters are easy and cheap to manufacture and are well suited for use in a degassing and filtration unit in a manufacturing line.

Although the above-mentioned US patent document No. 4,052,198 indicates significant improvements over previously known flue gas processes within a production line, there are still a number of unsolved problems. In order to achieve economic benefits and increased productivity, it is desirable to have access to degassing and filtration systems that can process molten metal continuously and at a processing rate that is consistent with practical stope technology. The use of known outgassing units in a production line, as described in the above-mentioned US patent no. 3,737,304, with the aim of achieving continuous outgassing and filtration, has, however, proven to be inefficient in operation, so that it will be required with a large number of treatment chambers to achieve adequate treatment of such molten metal quantities as are required for continuous casting processes. As a consequence of the treatment units' large scale, additional heating is required to prevent the molten metal being treated from breaking. Although certain improvements in the amount of metal that can be processed have been achieved by using a smaller processing system, such as the system described in US Patent No. 4,052,198, which prescribes the use of

ceramic filters and counter-flowing gas, such a system has nevertheless been found to have limited efficiency with regard to the amount of metal that can be processed, mainly as a result of the large pressure drops that exist when simultaneously counter-flowing the respective gas and metal through a filter body. As a consequence of this large pressure drop, a high-speed metal plume develops upstream of the filter element, so that it is required either a bending of the flow channel upstream of the filter element or a reduced supply of molten metal to

the treatment unit.

It is therefore a main purpose of the present invention to specify an improved method and produce an improved apparatus for degassing and filtering molten metal by using filter plates which are designed in such a way that a significant reduction in the pressure drop that occurs when gas and metal through the filter plates in mutually opposite flow directions.

It is a further purpose of the present invention to produce improvements as stated above which are simple and easy to use in practice, and which lead to very efficient outgassing and filtering of metal.

The invention thus relates to a method for degassing and filtering molten metal by passing the metal through at least one filter plate and by causing a flux gas to flow through the molten metal against the direction of flow of the metal melt, the flux gas inlet being positioned so that the released flux gas from the inlet passes through the filter plate.

The distinguishing feature of the method according to the invention is that the flux gas is made to flow through holes that are drilled through the filter plate and form preferred flow paths for the gas.

The invention also applies to an apparatus for degassing and filtering molten metal and which comprises at least one chamber with inlet and outlet for the metal and equipped with at least one filter plate, while at least one gas inlet for flux gas is arranged in such a way in relation to the filter plate that the discharged gas from the inlet passes through the filter plate, as the special feature of the device according to the invention lies in the fact that the filter plate has drilled holes throughout which constitute preferred flow paths for the flux gas through the filter plate.

In a preferred embodiment, several filter media are used which comprise an open cell structure which is made up of a large number of interconnected cavities and is preferably made of ceramic foam material where said cavities are surrounded by ceramic material. These filter media can comprise plates with sloping side surfaces designed to fit into corresponding sloping openings in said chamber walls. Resilient sealing material which is resistant to the molten metal is sealingly arranged on the beveled wall surfaces of said chamber when mounting the filter plates.

In accordance with a preferred embodiment of the device according to the present invention, the different filter media can have the same pore size and permeability or have different pore size and permeability, whereby the first medium has a relatively coarser pore structure, higher permeability and larger accessible flow surface than the second medium. In order to achieve the intended improvements according to the present invention, the first filter medium is equipped with a number of holes which are significantly larger than the pore size of the filter medium itself, in order thereby to produce a preferred flow path for the gas through the first filter medium. The flux gas is introduced on the underside of the first filter medium through a distribution plate placed in the chamber. This distribution plate is equipped with a large number of openings of a certain size and with a certain distribution in order thereby to reduce the diffusion distance for the present pollutants in gaseous form, while at the same time that mutual association of the flue gas bubbles emitted from the plate is mainly prevented.

According to the method of the invention, outgassing and filtering of molten metal takes place by passing the melt through a chamber in which the molten metal passes through at least two mutually separated filter media, the melt being brought into countercurrent contact with a flux gas while it is inside and on the upper side of the first filter medium, and the flux gas is fed through this first filter medium via a preferred flow path to thereby limit unwanted pressure drop over the first filter medium, so that more efficient gas treatment of the molten metal is achieved. The flux gas is supplied from at least one inlet opening arranged inside the chamber between the first and second filter medium, after which the gas is distributed and rises in contact with the metal melt inside the first filter medium.

The method of the invention can use a flux gas in the form of an inert gas, which preferably comprises a small proportion of active gas, such as chlorine or a halogen-saturated carbon compound. The gas used can be any suitable gas or gas mixture, such as nitrogen, argon, chlorine, carbon monoxide, freon 12 etc., which are known to give acceptable outgassing.

In the preferred embodiment for outgassing molten aluminum, mixtures of nitrogen and freon 12 or argon and freon 12 are preferably used. In addition, an overlying salt layer of alkali and alkaline earth chlorides as well as a fluoride can be applied to the surface of the metal melt as an aid during the degassing process by to prevent released pollutants in gaseous form from being taken up again by the surface of the melt. Typical salts that can be used are molten halides, such as sodium chloride, potassium chloride, magnesium chloride or mixtures of these compounds, the salts used being selected so that erosion of the degassing chamber's heat-resistant lining is reduced to a minimum. Alternatively, a covering layer in gaseous form can be used, e.g. of argon, nitrogen, etc., as a protective layer over the molten metal with the aim of reducing the return of released gas contaminants to the surface of the melt.

The present apparatus and method result in a considerable reduction in production when outgassing molten metal, as the outgassing takes place without interrupting the operation of the melting furnace. The construction of the apparatus also makes it possible to place it close to the spot, so that the possibilities for further contamination to penetrate the smelting are substantially reduced.

Application of the aforementioned first filter medium according to the present invention in the above-described apparatus makes it possible to arrange a preferred flow path for the gas through the filter medium, so that the pressure drop across the medium is reduced and thereby contributes to the apparatus gaining greater efficiency. Application of a distribution plate according to the present invention in the apparatus described above reduces the bubble size of the cleaning gas and at the same time increases the density of gas bubbles so that the effective adsorption surface of the bubbles increases for increased outgassing of the molten metal.

The increased outgassing efficiency according to the present invention also makes it possible to carry out the desired outgassing with a smaller amount of flux material, so that the gas emission as a result of the flux process is greatly reduced.

The present apparatus and method result in considerably increased productivity when outgassing molten metal, as the outgassing takes place without interrupting the alloying and melting processes. Furthermore, the design of the device allows it to be placed close to the stopping point. The present invention makes it possible to carry out a degassing and filtering process which entails a significant reduction in gas emissions of any kind compared to what normally exists in processes of this kind.

As a consequence of the use of easily replaceable filter media with carefully adapted filter properties, it will be possible with the method and apparatus of the invention to achieve such levels of purity for metal melts as have hitherto only been possible to achieve with the most extensive cleaning measures. Also, the use of a relatively coarse first filter medium to separate out larger entrained non-metallic particles before the metal melt reaches the second, finer filter, contributes to a large extent to increasing the effective lifetime of the latter filter. Furthermore, these high levels of purity are achieved by using filter media that require low production costs.

The invention will now be described in more detail with the help of the attached drawings, on which: Fig. 1 shows in section an apparatus embodiment according to the present invention seen from the side and in which the first and second filter media are arranged mainly side by side. Fig. 2 shows the distribution plate used in the apparatus in fig. 1, top view. Fig. 3 is a perspective sketch of the first filter medium with an interrupted part, to illustrate the present invention. Fig. 4 is a graphical representation showing the improved amount of strain per unit of time which is obtained in accordance with the method and apparatus according to the present invention. Fig. 5 shows, seen from the side, an alternative device version according to the present invention and in which said filter medium is arranged mainly horizontally.

In fig. 1 shows an apparatus arranged in a transfer system for molten metal and which may comprise discharge pans, discharge chutes, transfer chutes, metal processing crucibles and the like. The apparatus and the method according to the present invention can be used in many different places in the transfer system between the melting point and the melting point in the metal processing plant. Fig. 1 thus shows a heat-resistant degassing and filtering apparatus 10 which is divided by a partition wall 12 into chambers 14 and 16. The molten metal enters the chamber 14 through the inlet chute 18, passes under the partition wall 12 into the chamber 16 and from there onwards through outcome 20 for further treatment. The apparatus 10 can optionally be equipped with an inlet screen 22 which serves to delimit any salt layer 24 present on the surface of the molten metal in the chamber 14 in order to prevent this salt layer from flowing backwards along the chute 18.

In accordance with a preferred embodiment of the present invention, each of the chambers 14 and 16 is equipped with at least one peripheral edge, respectively. 26 and 28, which are mainly arranged next to each other and level with the bottom of the respective The inlet and outlet channels 18 and 20. The first peripheral edge 26 and the second peripheral edge 28 are shown in fig. 1 as a downward converging inclined surface which allows easy and convenient

installation and replacement of correspondingly designed filter media.

. Although the edges 26 and 28 are shown with beveled surfaces, the invention is not limited to this embodiment, as retaining edges can also be used with other means to hold the filter media according to the present invention in place, as will be indicated in the following.

Filter media 30 and 32 are arranged respectively. in chamber 14 and chamber 16, and may be provided with bevelled side edges 34 bevelled to match the correspondingly designed circumferential edges 26 and 28. The bevelled side edges 34 are provided with compliant sealing means 36 which are resistant to molten metal, and the respective filter media 30 and 32 together with the sealing means 36 are respectively placed in the chamber 14 and 16, in such a way that the sealing means 36 in both cases rest against the respective inclined surfaces along the edges 26 and 28.

The bottom of the heat-resistant degassing and filtering apparatus 10 is equipped in the chamber 14 with a stopped ceramic distribution plate 38 which is equipped with a large number of openings 40 for the introduction of a flux gas from an external source, which is not shown, so that the gas flows from the gas inlet 32 and the collection chamber 44 into the molten metal as it passes through the chamber 14.

In the preferred embodiment of the present invention, the use of a stopped ceramic distribution plate has a clear advantage over the usual methods and devices used for introducing flux gas into molten metal. In order to achieve the greatest possible efficiency in the outgassing process according to the invention, which means that the kinetic conditions of

. the adsorption reaction is made as effective as possible, the introduction of flux gas into the smelting should be carried out so that the smallest possible bubble size and the greatest possible bubble density are achieved, while at the same time the bubbles are avoided. The mean distance between the openings in the spacer plate must therefore be carefully adjusted so that the flow of flue gas bubbles is prevented while the diffusion distance that the gas contaminants must travel

through the smelting to reach a bubble, is reduced to a minimum. Maximum adsorption efficiency is achieved by using a distribution plate as shown in fig. 2. Use of mutually separated openings 40 in the distribution plate means that bubble accumulation is avoided and makes it possible to regulate the size and distribution of the bubbles. The size of the individual openings 40 determines the bubble size. In order to achieve the largest possible total surface for the intended adsorption reaction, the openings are consequently made as small as possible without the openings being plugged again by metal even after several times of use. In accordance with the present invention, opening sizes in the range 0.125 to 1.25 mm, preferably 0.25 to 0.750 mm, have been used for outgassing molten aluminum and aluminum alloys. The distance A between the openings, which is shown in fig. 2, is critical for achieving maximum dispersion of the bubbles while maintaining a sufficient distance between them to prevent the bubbles from merging into larger bubbles. Opening distances in the range of 6.25 to 125 mm, preferably 18.75 to 50 mm, have been found to be optimal values when outgassing molten aluminum and aluminum alloys.

The flux gases that can be used in connection with the present apparatus and method include many different well-known gas components, including chlorine gas and other halogen-containing gases, carbon monoxide as well as certain inert gas mixtures based on and including nitrogen, argon, helium and the like. A preferred gas mixture for use in accordance with the present invention in outgassing molten aluminum and aluminum alloys comprises a mixture of nitrogen or argon with dichlorodifluoromethane from about 2 to about 20% by volume, preferably 5 to 15% by volume. Together with this gas mixture, a molten salt mixture 24 can be used on the surface of the melt located inside the chamber 14, as this salt mixture can include halides such as sodium chloride, potassium chloride, magnesium chloride and mixtures of these compounds. It should be noted that the molten salt mixture can be selected so that reduced erosion of the flux chamber's heat-resistant lining is achieved. Furthermore, a protective gas atmosphere of argon, nitrogen or the like can be placed over the molten metal to thereby reduce the possibility of renewed adsorption of the released gas contaminants on the molten surface, in a similar way as with the molten salt. The material compositions indicated above are indicated for the sake of clarity and constitute no material limitation in connection with the present invention.

A preferred embodiment of the present invention comprises filter media with uniform, fine tolerance at significantly reduced costs. The filter medium therefore comprises a filter plate of the type shown in fig. 3. This filter plate 46 has an open cell structure which is characterized by a large number of interconnected cavities, so that the molten metal can pass through these and including the melt's content of entrained solid particles is removed or reduced from the stopped end product, or the exchange of gas contaminants is facilitated between the melt and a flux gas. Such a filter can e.g. include a fixed filter plate mounted in a sintered ceramic aggregate or a porous carbon plate. In the preferred embodiment, a filter of ceramic foam material of the type described in US Patent No. 3,962,081 is used. In accordance with this US patent document can . filters are made of ceramic foam material with an open cell structure with the distinctive feature that a large number of interconnected cavities are surrounded by a network of said ceramic material. The filter of ceramic foam material which is described in the above-mentioned US patent is particularly suitable for use according to the present invention, as it is inexpensive and can easily be used as a flow-through filter. This filter is also effective when filtering molten metal, particularly aluminium, as good filtering capacity is achieved at low costs under substantially different external circumstances. In accordance with the preferred embodiment, a first filter medium 30 can be produced with relatively coarse pore size with 2 to 8 pores per cm, and which has an air passage--7 2

necessity from 2,500 to 8,000 x 10 cm, while the other filter medium may comprise a relatively fine-pored filter with a pore count of from 8 to 18 pores per cm and an air permeability

-7 2

from 400 to 2,900 x 10 cm. The flow rate of the liquid metal through the filter can have a value from 12.5 to 125 cm 3 per

cm 2 filter surface per my. As pointed out earlier, both permeability and pore size for the respective filter media can of course be varied in accordance with the particular material being filtered, and the present invention is thus in no way limited to the value ranges which are only indicated as examples above.

The filter of ceramic foam material which is preferably used in the embodiment of the present invention is made of a flexible foam material with an open cell structure which is made up of a large number of interconnected cavities surrounded by a network of said flexible foam material, such as e.g. polyurethane

foam or cellulosic materials in foam form. The filter of ceramic foam material can be produced in accordance with the general process set out in US Patent No. 3,893,917, whereby an aqueous ceramic slurry is produced and the relevant foam material is impregnated with this so that the inner network of the foam material is coated with the ceramic slurry and the cavities are mainly filled with this material. The impregnated material is then compressed so that part of the slurry is driven out of the foam material and the remaining part is distributed evenly over the interior of the foam material. The thus coated foam material is then dried and heated to first burn out the flexible organic foam material and then sinter the ceramic coating, so that a baked-in ceramic foam material is obtained with a large number of interconnected cavities surrounded by a network of sintered ceramics in the same material form as the original flexible foam material; Many different ceramic materials can of course be chosen depending on the particular metal to be filtered. A mixture of aluminum oxide and chromium oxide is preferably used, but these materials can of course also be used separately or in combination with other ceramic materials. Other typical ceramic materials that can be used include zirconium oxide, magnesium oxide, titanium dioxide, silicon oxide and mixtures of these materials. Usually the slurry ceramic material contains from 10 to 40% water, as well as one or more rheological additives, binders or air curing agents.

As shown in fig. 3, the filter plate 46 may, according to the preferred embodiment, have a chamfered peripheral surface 48 designed to match corresponding chamfered edges in the wall of the filter chamber, as shown in fig. 1 and 4. Developmental variants can of course be used within the framework of the present invention, and a large range of variations of applicable geometric shapes can thus be imagined within the construction framework of the present apparatus, which is shown in fig. 2, but is not intended as a limitation of the scope of the invention.

In the event that the filter plate according to the present invention is designed to be thrown away after one use, it is important to produce effective means for sealing the filter plate in its holder in a way that allows easy assembly, disassembly and cleaning. The holder or filter chamber itself normally forms an integral part of a chute, outlet pan or crucible, etc., and should be made of hard-to-melt materials that are resistant to the molten metal and of the same type as used in normal chute constructions of the present kind. It is highly preferable that the filter is mounted tightly in working position by using flexible sealants or gaskets as shown and discussed earlier, and which surround the circumference of the filter plate along its chamfered surfaces. Gasket-type seals ensure leak-free assembly and also ensure that the filter medium can be easily removed, which is of significant importance for easy disassembly. In addition, if these sealants or gaskets prevent access of metal to the holder unit's sealing surfaces, the use of such means will significantly facilitate the cleaning of the holder unit and effectively extend its life by preventing metal attack. Due to its compliance, the gasket can also provide sufficient frictional force to hold the filter body in place in the holder or filter chamber without the need to use other types of retaining means. The yielding sealants must not be able to be wetted by the molten metal present, be able to resist chemical attack from this and be sufficiently heat-resistant to withstand the high operational

temperatures.

Plate-shaped filter units according to the present invention can be sealed by means of gaskets around their side edges and/or along the circumference of their larger side surfaces. The filter plate units according to the present invention are preferably sealed by means of an edge seal along the circumference of the filter plate, so that a positive seal is created and, when a gasket is used, a mechanical effect that holds the filter in place. In the event that a simple press fit is insufficient to hold the filter in place, various mechanical devices, such as wedges and retaining weights, can of course be used for this purpose.

Alternatively, the device 10 in fig. 1 be extended to be split at the edges 26 and 28, in a manner not shown, so that pressure can be applied to the seals by vice action when the separate units are joined. The bevel angle of the sealing surface in the filter chamber and the corresponding bevel angle of the filter plate will tend to form a positive seal and hold the filter in place against the buoyant forces acting on the filter. As indicated above, the gasket or seal used must of course be resistant to the molten metal used. Typical sealing materials used in the treatment of aluminum include heat-resistant fiber-type seals and many different compositions, such as the following illustrative seal types: (1) a seal comprising about 45% alumina, 52% silicon oxide, 1.3% ferric oxide and 1, 7% titanium oxide; (2) a seal containing about 55% silicon oxide, 45% alumina, 4% chromium oxide and 0.5% ferric oxide; and (3) a seal comprising about 53% silicon oxide, 46% aluminum oxide and 1% ferric oxide.

As indicated in fig. 1, it is a main feature of the present invention that a first filter medium 30 is arranged with a series of holes 50 which are substantially larger than the pore size of the filter itself. It has been observed that there are large pressure differences between the treatment zone for flue gas treatment downstream of the first filter medium and the corresponding area upstream of this filter medium, using the method and apparatus as stated in previously mentioned US patent document no. 4,052,198. This large pressure difference resulted from a buildup of flux gas on the underside of the filter plate. This difference constitutes a physical limit for the amount of flux gas that can be fed in countercurrent through the flow of molten metal, so that there is also a limit for the amount of molten metal that can be processed continuously and efficiently. In accordance with the present invention, it has been found that this pressure difference can be effectively regulated and reduced by equipping the filter plate with a series of holes that are significantly larger than the size of the filter's individual pores, so that a preferred flow path is created for the flux gas through the filter plate while essentially everything melted metal passes through the filter body. The reduced driving pressure difference achieved according to the present invention enables an increase in the amount of flux gas that can be introduced into the smelter, whereby an increase in the amount of molten metal that can be effectively processed is also achieved compared to the corresponding amount for known systems. The following embodiment example will further illustrate the present invention. A number of filter media of the ceramic foam type was produced in accordance with the process described in US Patent No. 3,893,917 for use as the first filter media in a reduced-scale test unit corresponding to the apparatus shown in fig. 1. Each filter was of size 12.5 x 12.5 x 2.5 cm and was designed to have an air permeability of 1750 x 10 -7 cm 2 , a porosity of 0.90 and a pore size corresponding to 12 pores per cm. 25 holes of the same size were drilled in each of the filters. It should be noted that instead of making the holes in the finished filter product by drilling or the like, the holes can be formed in the flexible foam material for the manufacturing process described in said US patent document no. 3,893,917. It is preferable that the holes are the same size and evenly distributed over the surface of the filter plate, so that the flux gas will be evenly distributed over the plate. The hole sizes in the filter were varied from 2.5 to 11.25 mm in steps of 0.25 mm. The filters were built into the sample unit on a reduced scale with the aim of studying the flow characteristics of gas and liquid in countercurrent during a filtration and degassing process. Liquid metal was introduced into the filtration and degassing chamber in flow rates of 15,150, 22,730 and 30,300 cm 3 per my. The gas flow was adjusted so that a pressure drop corresponding to a level difference of 1.27 cm between the inlet and outlet channels was created, as shown by the letter B in fig. 1. The results obtained are graphically illustrated in fig. 4.

As will be seen from fig. 4 allows the rows of boreholes through the filter medium according to the invention to introduce a larger amount of flux gas into the metal melt, so that a more effective filtering and degassing apparatus is obtained than that previously known.

By arranging a preferred flow path for the flue gas through the filter plate, the pressure difference across the plate is reduced so that a corresponding lowering of the level difference between the device inlet and outlet is achieved, which in turn enables the use of correspondingly smaller device units at the same time as the degassing efficiency increases. It has been found that hole sizes up to 11.25 mm can be effectively used without the filtering effect of the filter plate being reduced to a significant extent.

Fig. 5 shows an alternative device embodiment according to the invention, in which the filter plates are arranged mainly horizontally. The apparatus 50 is equipped with an inlet chute 52 which leads to the chamber 54 where the filtration and degassing take place. The chamber 54 is shown to be mainly bowl-shaped and is arranged with its bottom outlet below the level of the inlet chute 52, so that the molten metal which is supplied will flow downwards through the filter medium. The special feature of the chamber 54 is that it is equipped with at least two peripheral edges 56 and 58. The first peripheral edge 56 is placed in the upper part of the chamber 54 and is arranged in the same way as in fig. 1 at the level of the bottom of the inlet chute 52. The other peripheral edge 58 is as indicated in fig. 5 placed in such a way inside the chamber 54 that it effectively divides the chamber into two parts 60 and 62. Like the first peripheral edge 56, the second peripheral edge 58 in fig. shown with downwardly directed converging bevels which enable easy and simple installation and replacement of suitably designed filter media. Although the edges 56 and 58 are shown with inclined surfaces, the present invention is in no way limited to this embodiment, as edges equipped with other means to keep the filter medium in place can also be used within the scope of the invention, as will be described in more detail below. The edge 58 is shown to have a reduced extent in relation to the first peripheral edge 56 to allow unhindered handling of the filter medium within this peripheral edge.

The first partial chamber 60 comprises the area between the first and second filter media, which respectively are denoted by 64 and 66. As shown in the figure, the aforementioned filter media 64 and 66 can also have chamfered peripheral surfaces 68 which are extended to fit together with the correspondingly designed peripheral edges 56 and 58. The chamfered peripheral surfaces 68 in the assembled position are equipped with yielding sealants 70 which are resistant to molten metal, and the respective filter media 64 and 66 with continued sealants 70 are introduced in sequence into the interior of the chamber 54, so that the sealants 70 in each case come into contact with the respective beveled peripheral edges 56 and 58.

As stated above, the peripheral edges 56 and 58, by supporting their respective filter media 64 and 66, divide the chamber 54

in the partial chamber 60 and 62. Reference must now be made again to fig. 5 where it is shown that the partial chamber 60 in accordance with the present invention is equipped with at least one inlet opening 72 which forms one or more mouths for a flow channel 74 or manifold for supplying a flux gas to the smelter from an external source which is not shown.

In accordance with the present invention, the first filter medium 64 is equipped with a number of holes 76 to thereby form a preferred flow path for the flux gas. It is understood that the flue gas can be supplied through a distribution plate of the type shown and described with reference to fig. 1.

There are many different cases where the method and apparatus of the invention can be used in all the different embodiments indicated above. Especially in the case of a continuous stopping process, a pair of flux filtration chambers can be suitably used in parallel. In such a process, the plant's large length and the corresponding total metal flow may require that the filter media be replaced during operation. Such exchanges can be facilitated by the use of parallel flow channels each containing a filtration chamber, together with means for transferring the metal flow from one channel to the other, e.g. by means of valves, screens or the like. The metal flow will thus be limited to one filtering chamber at a time, and diverted to the second channel as soon as the level drop above the first filtering chamber increases above a certain value. It will be realized that such a switching process will ensure the supply of an end-lb stream of filtered metal to a continuous stopping process. 1 addition to what is stated above, the present apparatus and method according to the invention will be able to be modified in many different ways within the framework of the present invention for adaptation to different operating processes. In the case where small, individual portions of molten metal are to be processed and stamped, it is e.g. It is desirable that the different filter media can be kept in operation for several such individual portions. For this purpose, the filter media may be drawn somewhat downwards from the levels of the through-flow passage and the discharge chute, so that the remaining melt will fill the chamber and cover both filters after the flow of melt has ceased. In accordance with this modified embodiment, at least one cover unit can be used which can be placed on the upper side of the remaining melt and is equipped with heat sources such as a number of heat radiating elements to keep the melt in a liquid state.

Other modifications are also conceivable within the scope of the present invention, including the arrangement of a number of inlet openings around the circumference of the various chambers immediately below the respective first filter media. In addition and without it being shown here, the inlet openings can be extended to the middle area of said chambers with the extension of the respective supply channels, so that the flux gas can penetrate into the melt from a point in the middle of the chamber. Both the arrangement of a manifold of flue gas inlet and one or more inlet openings arranged inside the chamber include modifications which in themselves can be subject to varying constructions, and the invention is thus not limited to particular designs.

see in this regard.

Claims (9)

1. Procedure for degassing and filtering molten metal by passing the metal through at least one filter plate (3 0, 3 2) as well as by bringing a flux gas to flow through the molten metal against the flow direction of the metal melt, the flux gas inlet (3 8, 40, 44) being placed so that the flux gas emitted from the inlet passes through the filter plate (3 0), characterized in that the flux gas is brought to flow through holes (50) which are drilled through the filter plate (30) and form preferred flow paths for the gas. 2. Apparatus for degassing and filtering molten metal and which comprises at least one chamber (14, 16) with inlet (18) and outlet (20) for the metal and equipped with at least one filter plate (3 0, 3 2), while at least one gas inlet (3 8, 40, 44) for flux gas is arranged in such a way in relation to the filter plate that the emitted gas from the inlet passes through the filter plate, characterized in that the filter plate (3 0) has holes drilled throughout ( 50) which constitute preferred flow paths for the flux gas through the filter plate (30). 3. Apparatus as stated in claim 2, characterized in that the holes (50) have the same diameter which is significantly larger than the filter plate's pore size. 4. Apparatus as specified in claim 2 or 3, characterized in that said hole (50) has a diameter in the range from 2.5 to 11.25 mm. 5. Apparatus as set forth in claims 2-4, characterized in that the flux gas inlet (42) comprises a distribution plate (3 8) which is equipped with a number of openings (40) of such an adapted size and mutual distance that the size and spread of flux gas bubbles that are emitted through the openings (40) is mainly in accordance with the size and distribution of the drill holes (50) in the filter plate (30). 6. Apparatus as stated in claim 5, characterized in that the diameter of the openings (40) is in the range 0.125 to 1.25 mm, preferably in the range 0.25 to 0.50 mm. 7. Apparatus as stated in claim 5 or 6, characterized in that the distance between the openings (40) is in the range 6.25 to 125 mm, preferably in the range from 18.75 to 50 mm. 8. Apparatus as stated in claims 2-7, characterized in that the filter plate (30) is made of ceramic material with an open cell structure and a large number of interconnected cavities and pierced by said holes (50) to form preferred flow paths through the plate. 9. Apparatus as stated in claim 8, characterized in that the filter plate has an air permeability in the range 400 to 8,000 x IO<-7> cm2, a porosity of 0.80 to 0.95 and a pore size corresponding to 8 to 18 pores per cm.